Drop $(srcdir)/ prefix analogous to before the file (and rule) movement
and move it outside of the NEED_RADEON_LLVM conditional.
Otherwise the build may fail as below.
make[3]: *** No rule to make target 'common/sid_tables.h', needed by 'distdir'. Stop.
Fixes: b838f64237 "ac/debug: Move sid_tables.h generation to common
code."
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>