fix compile error with glibc toolchains

test/nptl/Makefile.in

@@ -100,6 +100,7 @@ LDFLAGS_tst-fini1 = -Wl,-rpath=./ tst-fini1mod.so
 LDFLAGS_tst-fini1mod.so = -Wl,-soname=tst-fini1mod.so
 LDFLAGS_tst-unload = -ldl
 LDFLAGS_tst-cancel5 := -lpthread
+LDFLAGS_tst-cancel18 := -lrt
 LDFLAGS_tst-cancel23 := -lc -lpthread
 LDFLAGS_tst-vfork1x := -lc -lpthread
 LDFLAGS_tst-vfork2x := -lc -lpthread