Fix build with -Wwrite-strings.

Change-Id: If76154c8d255600e925a408acdc674fc7dad0359
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://boringssl-review.googlesource.com/7526
Reviewed-by: David Benjamin <davidben@google.com>

crypto/evp/print.c

@@ -136,8 +136,7 @@ static void update_buflen(const BIGNUM *b, size_t *pbuflen) {
 
 static int do_rsa_print(BIO *out, const RSA *rsa, int off,
                         int include_private) {
-  char *str;
-  const char *s;
+  const char *s, *str;
   uint8_t *m = NULL;
   int ret = 0, mod_len = 0;
   size_t buf_len = 0;