Include limits.h where needed

This will be needed when TF-PSA-Crypto's `build_info.h` stops including
`limits.h`, which it currently does by accident because it includes
`check_config.h` which wants `limits.h` to check `CHAR_BIT`.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 6e9ebf9..c80a3f5 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -16,6 +16,7 @@
 
 #include "mbedtls/build_info.h"
 
+#include <limits.h>
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else