add missing <stdio.h> include to unit tests for printf

This fixes the build with --without-openssl on musl. glibc worked
previously because it got stdio.h implicitly through resolv.h.

Author: michaelforney

regress/unittests/kex/tests.c

@@ -3,6 +3,10 @@
  * Placed in the public domain
  */
 
+#include "includes.h"
+
+#include <stdio.h>
+
 #include "../test_helper/test_helper.h"
 
 void kex_tests(void);

regress/unittests/sshkey/tests.c

@@ -7,6 +7,8 @@
 
 #include "includes.h"
 
+#include <stdio.h>
+
 #include "../test_helper/test_helper.h"
 
 void sshkey_tests(void);