port_fwd, util: Include additional headers to fix build with musl

lseek() is declared in unistd.h, and stdio.h provides sscanf().
Include these two headers in port_fwd.c.

SIGCHLD, even if used exclusively for clone(), is defined in
signal.h: add the include to util.h, as NS_CALL needs it.

Reported-by: lemmi <lemmi@nerd2nerd.org>
Link: 1903952660 (step):7:57
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

util.h

@@ -10,6 +10,7 @@
 #include <stdarg.h>
 #include <stdbool.h>
 #include <string.h>
+#include <signal.h>
 
 #include "log.h"