glsl: Add pthread libs to cache_test

glsl: Add pthread libs to cache_test

Fixes the following compile error, present when the SHA1 library is libgcrypt:

  CCLD     glsl/tests/cache-test
glsl/.libs/libglsl.a(libmesautil_la-mesa-sha1.o): In function `call_once':
/mesa/src/util/../../include/c11/threads_posix.h:96: undefined reference to `pthread_once'

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
(cherry picked from commit 5c73ecaac487eba36e15f22be2e9396c4a0ffe46)