Adds stdlib.h header to dh_genprime sample app

Use of the atoi() function requires stdlib.h to be included when
MBEDTLS_PLATFORM_C is not defined.

programs/pkey/dh_genprime.c

@@ -29,6 +29,7 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
+#include <stdlib.h>
 #define mbedtls_printf     printf
 #define mbedtls_time_t     time_t
 #endif