glsl/linker: Don't include interface name for built-in blocks

glsl/linker: Don't include interface name for built-in blocks

Commit 11096ec introduced a regression in some piglit tests (e.g.,
arb_program_interface_query-resource-query).  I did not notice this
regression because other (unrelated) problems caused failed assertions
in those same tests on my system... so they crashed before getting to
the new failure.

v2: Use is_gl_identifier.  Suggested by Tim.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: mesa-stable@lists.freedesktop.org