.gitignore: Qualify the path for the ignored build directory.

This was origally added for scons, which is gone, but keep it around since
enough people use "build" for their meson builds that you probably
shouldn't add anything to git under that name.  The qualifying '/' is
needed because we have a .gitlab-ci/build/ directory where we do check in
code.

Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16827>

.gitignore

@@ -1,4 +1,4 @@
 *.pyc
 *.pyo
 *.out
-build
+/build