glsl: Link tests with CLOCK_LIB.

Fix 'make check' linking errors with glibc < 2.17.

  CXXLD  glsl/glsl_test
glsl/.libs/libglsl.a(libmesautil_la-u_queue.o): In function `u_thread_get_time_nano':
src/util/../../src/util/u_thread.h:84: undefined reference to `clock_gettime'

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
1 file changed