Fix warning with MD/SHA ALT implementation

fixes #239

library/sha512.c

@@ -52,13 +52,13 @@
 #endif /* MBEDTLS_PLATFORM_C */
 #endif /* MBEDTLS_SELF_TEST */
 
+#if !defined(MBEDTLS_SHA512_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;
 }
 
-#if !defined(MBEDTLS_SHA512_ALT)
-
 /*
  * 64-bit integer manipulation macros (big endian)
  */