Make the secp256k1_declassify argument constant

Summary:
This is required to declassify pointers to constant memory. Declassify should
never modify its argument.

This is a partial backport of secp256k1 [[bitcoin-core/secp256k1#558 | PR558]] : bitcoin-core/secp256k1@3e08b02

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7636

src/secp256k1.c

@@ -227,7 +227,7 @@ void secp256k1_scratch_space_destroy(const secp256k1_context *ctx, secp256k1_scr
  *  of the software. This is setup for use with valgrind but could be substituted with
  *  the appropriate instrumentation for other analysis tools.
  */
-static SECP256K1_INLINE void secp256k1_declassify(const secp256k1_context* ctx, void *p, size_t len) {
+static SECP256K1_INLINE void secp256k1_declassify(const secp256k1_context* ctx, const void *p, size_t len) {
 #if defined(VALGRIND)
     if (EXPECT(ctx->declassify,0)) VALGRIND_MAKE_MEM_DEFINED(p, len);
 #else