missing_resource.h: RLIMIT_RTTIME is defined since glibc-2.14

Now our baseline is glibc-2.31.

src/test/test-rlimit-util.c

@@ -8,7 +8,6 @@
 #include "alloc-util.h"
 #include "capability-util.h"
 #include "macro.h"
-#include "missing_resource.h"
 #include "process-util.h"
 #include "rlimit-util.h"
 #include "string-util.h"