automake: get in-tree `make distclean' working again.

automake: get in-tree `make distclean' working again.

With earlier commit we've handled the `make distclean' out of tree
build, yet we failed to attribute that for in-tree builds the test
condition will return 1. Thus effectively the target will be considered
as "failed".

Fixes: b7f7ec78435 ("mesa: automake: distclean git_sha1.h when building
OOT")
Cc: <mesa-stable@lists.freedesktop.org>
Tested-by: Andy Furniss <adf.lists@gmail.com>
Reported-by: Andy Furniss <adf.lists@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>