Remove superfluous include from zeroize.c

This brings zeroize.c and test_zeroize.gdb in sync with development.

The include was introduced in 3b0c43063 (#2710).
Reverts ff8ae1115 from the same pull request.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

programs/test/zeroize.c

@@ -40,7 +40,6 @@
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
-#include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf     printf
 #define mbedtls_exit       exit