Fix warning with MD/SHA ALT implementation

fixes #239

library/md2.c

diff --git a/library/md2.c b/library/md2.c
index 3263a22..88d679f 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -47,13 +47,13 @@
 #endif /* MBEDTLS_PLATFORM_C */
 #endif /* MBEDTLS_SELF_TEST */
 
+#if !defined(MBEDTLS_MD2_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_MD2_ALT)
-
 static const unsigned char PI_SUBST[256] =
 {
     0x29, 0x2E, 0x43, 0xC9, 0xA2, 0xD8, 0x7C, 0x01, 0x3D, 0x36,