Fix $? when dumping core. (Closes: #509041)

WCOREDUMP is in <sys/wait.h>

Without this, $? & 128 doesn't get set properly on some (glibc) systems
when dumping core.
diff --git a/perl.h b/perl.h
index abcae45..ebf57cd 100644
--- a/perl.h
+++ b/perl.h
@@ -663,6 +663,11 @@ register struct op *Perl_op asm(stringify(OP_IN_REGISTER));
 #   include <unistd.h>
 #endif
 
+/* for WCOREDUMP */
+#ifdef I_SYS_WAIT
+#   include <sys/wait.h>
+#endif
+
 #ifdef __SYMBIAN32__
 #   undef _SC_ARG_MAX /* Symbian has _SC_ARG_MAX but no sysconf() */
 #endif