Add missing sidestaketablemodel.cpp entry in CMakeLists.txt #1751

Job	Run time
macOS 11 native [GOAL install] [GUI] [no depends]	22m 8s
Lint	1m 6s
ARM [GOAL install] [buster]	15m 39s
Win32	19m 23s
Win64	23m 48s
i386 Linux [GOAL install] [focal]	18m 19s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	27m 23s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	26m 2s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	19m 45s
	2h 53m 33s

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