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/ripemd160.c b/library/ripemd160.c
index 2ba48b7..6cf027f 100644
--- a/library/ripemd160.c
+++ b/library/ripemd160.c
@@ -34,6 +34,7 @@
 #if defined(MBEDTLS_RIPEMD160_C)
 
 #include "mbedtls/ripemd160.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -71,11 +72,6 @@
 }
 #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;
-}
-
 void mbedtls_ripemd160_init( mbedtls_ripemd160_context *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_ripemd160_context ) );