Avoid implicit function declarations in tests, for C99 compatibility (#521)

Include <stdio.h> for printf and define _GNU_SOURCE for execvpe.

Future compilers will not support implicit function declartions by
default, causing these tests to fail to build.

Author: fweimer-rh

test/cases/intercept/preload/errno_reset.c

@@ -5,6 +5,7 @@
 #include <stdlib.h>
 #include <errno.h>
 #include <string.h>
+#include <stdio.h>
 
 int main()
 {

test/cases/intercept/preload/posix/execvpe/success.c

@@ -8,6 +8,7 @@
 #include "config.h"
 
 #if defined HAVE_UNISTD_H
+#define _GNU_SOURCE
 #include <unistd.h>
 #endif