Merge bitcoin-core/secp256k1#1366: field: Use restrict consistently…

… in fe_sqrt

b79ba8a field: Use `restrict` consistently in fe_sqrt (Tim Ruffing)

Pull request description:

  That is, use it also in the definition and not only the declaration.

  I believe this was the intention of commit
  bitcoin-core/secp256k1@be82bd8, but it was omitted there.

  edit: Changed the description. I'm not entirely sure but after looking at the standard, I tend to think this is more than a cosmetic change, and only this change actually makes the parameters `restrict`. Anyway, I believe making them `restrict` was simply forgotten in be82bd8.

ACKs for top commit:
  sipa:
    utACK b79ba8a

Tree-SHA512: eecec7674d8cef7833d50f4041b87241ca8de4839aa8027df1c422b89f5a1bcef3916ac785057a596c459ce1aa9d41e5a21ecb6fed9c5d15a1d9f588c7ee208e

src/field_impl.h

@@ -44,7 +44,7 @@ SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const
     return secp256k1_fe_normalizes_to_zero_var(&na);
 }
 
-static int secp256k1_fe_sqrt(secp256k1_fe *r, const secp256k1_fe *a) {
+static int secp256k1_fe_sqrt(secp256k1_fe * SECP256K1_RESTRICT r, const secp256k1_fe * SECP256K1_RESTRICT a) {
     /** Given that p is congruent to 3 mod 4, we can compute the square root of
      *  a mod p as the (p+1)/4'th power of a.
      *