glsl: Link glsl_compiler with CLOCK_LIB.

Fix linking error on CentOS 6.

  CXXLD  glsl_compiler
glsl/.libs/libstandalone.a(lt16-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>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/compiler/Makefile.glsl.am

@@ -147,7 +147,8 @@ glsl_compiler_SOURCES = \
 	glsl/main.cpp
 
 glsl_compiler_LDADD = \
-	glsl/libstandalone.la
+	glsl/libstandalone.la \
+	$(CLOCK_LIB)
 
 glsl_glsl_test_SOURCES = \
 	glsl/test.cpp \