git_sha1.c is generated in $(top_builddir)/src.
Fixes out-of-tree builds since 4825264f75.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96516
Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>