Add missing `#include` for `rlim_t`

My local build in the shell was failing while CI was fine; not sure why
that is but having the include here is definitely more correct.

Per the POSIX spec, this is where it is supposed to be gotten
https://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys/resource.h.html

src/libutil/current-process.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include <optional>
+#include <sys/resource.h>
 
 #include "types.hh"