util: Fix build on old glibc.

We need to link librt for u_thread.h's clock_gettime() call.

Fixes: b822d9dd67 ("gallium/util: move u_queue.{c,h} to src/util")
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/util/Makefile.am

@@ -46,7 +46,9 @@ libmesautil_la_SOURCES = \
 	$(MESA_UTIL_FILES) \
 	$(MESA_UTIL_GENERATED_FILES)
 
-libmesautil_la_LIBADD = $(ZLIB_LIBS)
+libmesautil_la_LIBADD = \
+	$(CLOCK_LIB) \
+	$(ZLIB_LIBS)
 
 libxmlconfig_la_SOURCES = $(XMLCONFIG_FILES)
 libxmlconfig_la_CFLAGS = \