add a couple missing consts to ecmult_pippenger_wnaf

Author: apoelstra

src/ecmult_impl.h

@@ -631,7 +631,7 @@ struct secp256k1_pippenger_state {
  * to the point's wnaf[i]. Second, the buckets are added together such that
  * r += 1*bucket[0] + 3*bucket[1] + 5*bucket[2] + ...
  */
-static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_window, struct secp256k1_pippenger_state *state, secp256k1_gej *r, secp256k1_scalar *sc, secp256k1_ge *pt, size_t num) {
+static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_window, struct secp256k1_pippenger_state *state, secp256k1_gej *r, const secp256k1_scalar *sc, const secp256k1_ge *pt, size_t num) {
     size_t n_wnaf = WNAF_SIZE(bucket_window+1);
     size_t np;
     size_t no = 0;