Only generates documentation for explicit input

Excluding the files list made it hard to find documentation. However,
unsetting this value caused all the headers, even the private ones, to
be present in the list. While annoying, explicitly specifying input gets
(my) desired result.

Doxyfile

@@ -4,4 +4,4 @@ TYPEDEF_HIDES_STRUCT   = YES
 EXTRACT_PRIVATE        = YES
 HIDE_UNDOC_MEMBERS     = YES
 HIDE_UNDOC_CLASSES     = YES
-SHOW_FILES             = NO
+INPUT                  = grade_hueco.h