Adjust function definition in drm_fb_helper.c to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

sys/dev/drm2/drm_fb_helper.c:86:18: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
framebuffer_alloc()
                 ^
                  void

This is because framebuffer_alloc() is declared with a (void) argument
list, but defined with an empty argument list. Make the definition match
the declaration.

MFC after: 3 days