Add needed #include.

Per POSIX, WIFSIGNALED and related macros are provided by <sys/wait.h>.
Apparently on Linux they're also pulled in by some other inclusions,
but BSD-ish systems are pickier.  Fixes portability issue in ffa4cbd6.

Per buildfarm.

src/backend/commands/copy.c

@@ -17,6 +17,7 @@
 #include <ctype.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <sys/wait.h>
 
 #include "access/heapam.h"
 #include "access/htup_details.h"