omx: fix build with gcc 10

omx: fix build with gcc 10

bellagio/omx header files reference a global variable without the
extern keyworkd.
Now that gcc-10 enables the '-fno-common' by default the build fails.
Since these are external headers we can't easily fix them, so for
now build the omx module with the '-fcommon' flag to keep the
previous behavior.

See https://gitlab.freedesktop.org/mesa/mesa/issues/2385

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4058>