src/meta-openembedded:libusbgx: uprev to the latest commit

Uprev libusbgx to 721e3a1 to get following fixes:
```
721e3a1 correct attribute import / export
bbdaad2 Document the license of the library and examples
36e71e1 Rename the net class variables to class_
efdd7a6 tests: update test code with new net attributes
4ca4a76 net: adds class, subclass and protocol attributes
b1c8586 Merge pull request #64 from mgrzeschik/master
8b91dbb uvc: add isoc parameter settings
50fe6a7 uvc: fix init_formats function
afec0d6 uvc: fix exporting attrs with layouts missing a format
0607844 Merge pull request #63 from mgrzeschik/uvc-fix
20d7fe2 uvc: fix unsigned nmb to int
ed1fb6d Merge pull request #62 from linux-usb-gadgets/uvc-fix
157cbd8 uvc: fix return check on scandir
741230b Merge pull request #61 from linux-usb-gadgets/uvc-libconfig-import-export-support
53231c7 uvc: add import/export support
61a4186 uvc: consistently use defines for common strings
57dc1d5 uvc: check attr pointer in set_attrs
4061ffd uvc: remove unnedded USBG_UVC_MAX_PATH_LENGTH define
1c94fb8 uvc: remove unused variable
b28dd77 uvc: fix compile warnings
6bd989c gadget-uvc: fix 4k format resolution
d0f2f4f Revert "Use an unnamed variable instead of a named one"
92deb97 pkgconfig: make Requires for libconfig optional
38dd92f gadget-vid-pid-remove: re-split default vendor/product defines from variables
fa4e7e9 Use an unnamed variable instead of a named one
1ee53e4 Copy ether_addr structs to avoid union pointer alignment issues
4852080 Delete embedded copies of libtool m4 files
ccba65f Tell git which paths to ignore are directories
3a0f66a Tell git to ignore autotools cruft in the src functions subdirectory
37fcf90 Tell git to ignore generated usbg_version.h header
1e7161d Tell git to ignore the generated cmake config file
b9a5b9c Tell git to ignore the right generated pkg-config .pc file
e972b9e Document that the configfs Linux kernel module needs to be loaded
97b6477 Support printer function
ad378b4 Fix --without-libconfig segment fault on gadget-hid attr setting
67605a0 libusbgx: examples: gadget-vid-pid-remove: fix exit code on success
e0cdec8 Merge pull request #54 from mgrzeschik/gadget-vid-pid-remove
547a759 Merge pull request #55 from mgrzeschik/uvc
52aa5ed libusbgx: Add UVC support
852087a libusbgx: examples: gadget-vid-pid-remove: add dynamic vid pid support
af2292c Fix typo in comment
664b7e6 Added support for CMake projects macros
b5ad66e Fix #39 Compilation fails on gcc v8
283879d libusbgx: fix --without-libconfig build against glibc-2.28
```

Also add two pending MRs from github:
```
linux-usb-gadgets/libusbgx#72
linux-usb-gadgets/libusbgx#73
```

Signed-off-by: Ming Liu <[email protected]>
Signed-off-by: Khem Raj <[email protected]>