mesa: automake: distclean git_sha1.h when building OOT

mesa: automake: distclean git_sha1.h when building OOT

In the case of out-of-tree (OOT) builds, in particular when building
from tarball, we'll end up with the file in both srcdir and builddir.

We want the former to remain intact (since we need it on rebuild) while
the latter should be removed otherwise `make distclean' gets angry at
us.

Ideally there'll be a solution that feels a bit less of a hack. Until
then this does the job exactly as expected.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>