Fix includes in udp_proxy.c

The program uses atoi() unconditionally, so it should include stdlib.h
unconditionally. Previously this happened to be indirectly included by
some other header (via pk.h via ssl.h) but we should not rely on that.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

programs/test/udp_proxy.c

@@ -17,11 +17,11 @@
 #include "mbedtls/build_info.h"
 
 #include <limits.h>
+#include <stdlib.h>
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
-#include <stdlib.h>
 #if defined(MBEDTLS_HAVE_TIME)
 #include <time.h>
 #define mbedtls_time            time