libutil++: Include <cerrno> in stringf.cc

stringf.cc uses errno and related macros without including <cerrno>.

Their availability is guaranteed only when the corresponding header
is included; transitive exposure is implementation-defined.

Modern libc++ has been progressively reducing incidental transitive
includes as part of its header removal policy (see LLVM libc++ Header
Removal Policy and D132284), making such dependencies brittle.

This change includes <cerrno> explicitly to make the dependency
well-defined. No functional or behavioural change intended.

Approved by:	fuz
Signed-off-by:	Faraz Vahedi <[email protected]>
Pull-Request:	https://github.com/freebsd/freebsd-src/pull/2188

lib/libutil++/libutil++.hh

@@ -13,6 +13,7 @@
 #include <netdb.h>
 #include <unistd.h>
 
+#include <cerrno>
 #include <cstdarg>
 #include <cstdio>
 #include <cstdlib>