Remove individual copies of mbedtls_zeroize()

This commit removes all the static occurrencies of the function
mbedtls_zeroize() in each of the individual .c modules. Instead the
function has been moved to utils.h that is included in each of the
modules.

library/gcm.c

@@ -38,6 +38,7 @@
 #if defined(MBEDTLS_GCM_C)
 
 #include "mbedtls/gcm.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -80,11 +81,6 @@
 }
 #endif
 
-/* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
-}
-
 /*
  * Initialize a context
  */