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.
diff --git a/library/xtea.c b/library/xtea.c
index fe0a350..65b4165 100644
--- a/library/xtea.c
+++ b/library/xtea.c
@@ -28,6 +28,7 @@
 #if defined(MBEDTLS_XTEA_C)
 
 #include "mbedtls/xtea.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -42,11 +43,6 @@
 
 #if !defined(MBEDTLS_XTEA_ALT)
 
-/* 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;
-}
-
 /*
  * 32-bit integer manipulation macros (big endian)
  */