glsl/linker: Reduce scope of non-flat integer varying fix.

glsl/linker: Reduce scope of non-flat integer varying fix.

In the mailing list discussion of "glsl/linker: fix varying packing
for non-flat integer varyings." (commit 7862bde), we concluded that
since the bug only applies to integral variables, it is safer to just
apply the bug fix to integer varyings.  I forgot to make the change
before pushing the patch upstream.  (Note: we aren't aware of any bugs
in commit 7862bde; it just seems wise to be on the safe side).

This patch makes the change.  Assuming commit 7862bde gets
cherry-picked back to 9.1, this commit should be cherry-picked too.

NOTE: This is a candidate for the 9.1 release branch.