gl/tests: fix unused-but-set warning if built with gles2-only
../tests/check/pipelines/gl-launch-lines.c:314:12: error: variable 'have_gldifferencematte' set but not used [-Werror,-Wunused-but-set-variable]
gboolean have_gldifferencematte;
^
1 error generated.
Edited by Matthew Waters