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>

src/compiler/Makefile.glsl.am

@@ -62,8 +62,11 @@ glsl_tests_blob_test_LDADD =				\
 
 glsl_tests_cache_test_SOURCES =				\
 	glsl/tests/cache_test.c
+glsl_tests_cache_test_CFLAGS =				\
+	$(PTHREAD_CFLAGS)
 glsl_tests_cache_test_LDADD =				\
-	glsl/libglsl.la
+	glsl/libglsl.la					\
+	$(PTHREAD_LIBS)
 
 glsl_tests_general_ir_test_SOURCES =			\
 	glsl/tests/builtin_variable_test.cpp		\