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/pkparse.c b/library/pkparse.c
index 5ad5edf..093ef58 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -30,6 +30,7 @@
 #include "mbedtls/pk.h"
 #include "mbedtls/asn1.h"
 #include "mbedtls/oid.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -60,14 +61,6 @@
 #define mbedtls_free       free
 #endif
 
-#if defined(MBEDTLS_FS_IO) || \
-    defined(MBEDTLS_PKCS12_C) || defined(MBEDTLS_PKCS5_C)
-/* 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;
-}
-#endif
-
 #if defined(MBEDTLS_FS_IO)
 /*
  * Load all data from a file into a given buffer.