extmod/crypto: Add static keyword where it should be.

extmod/crypto-algorithms/sha256.c

@@ -41,7 +41,7 @@ static const WORD k[64] = {
 };
 
 /*********************** FUNCTION DEFINITIONS ***********************/
-void sha256_transform(SHA256_CTX *ctx, const BYTE data[])
+static void sha256_transform(SHA256_CTX *ctx, const BYTE data[])
 {
 	WORD a, b, c, d, e, f, g, h, i, j, t1, t2, m[64];