Suppress `-Wunused-parameter` when building for coverage analysis #1244

On master (427bc3cdcfbc74778070494daab1ae5108c71368), when configured with --enable-coverage, gcc emits multiple warnings:

$ make
...
src/ecmult_impl.h: In function ‘secp256k1_ecmult_table_get_ge’:
src/ecmult_impl.h:122:113: warning: unused parameter ‘w’ [-Wunused-parameter]
  122 | SECP256K1_INLINE static void secp256k1_ecmult_table_get_ge(secp256k1_ge *r, const secp256k1_ge *pre, int n, int w) {
      |                                                                                                             ~~~~^
src/ecmult_impl.h: In function ‘secp256k1_ecmult_table_get_ge_lambda’:
src/ecmult_impl.h:132:143: warning: unused parameter ‘w’ [-Wunused-parameter]
  132 | SECP256K1_INLINE static void secp256k1_ecmult_table_get_ge_lambda(secp256k1_ge *r, const secp256k1_ge *pre, const secp256k1_fe *x, int n, int w) {
      |                                                                                                                                           ~~~~^
src/ecmult_impl.h: In function ‘secp256k1_ecmult_table_get_ge_storage’:
src/ecmult_impl.h:142:129: warning: unused parameter ‘w’ [-Wunused-parameter]
  142 | SECP256K1_INLINE static void secp256k1_ecmult_table_get_ge_storage(secp256k1_ge *r, const secp256k1_ge_storage *pre, int n, int w) {
      |                                                                                                                             ~~~~^
...

This PR fixes them.

Can confirm that this fixes automake.

Concept ACK

What about replacing the macro by a function, similar to https://github.com/bitcoin-core/secp256k1/blob/master/src/field_5x52_impl.h#L37? Macros are ... fragile.

What about replacing the macro by a function, similar to https://github.com/bitcoin-core/secp256k1/blob/master/src/field_5x52_impl.h#L37? Macros are ... fragile.

Done.

utACK 5bb03c29116409ace8855e64bf2e2b2d45871469

ACK 5bb03c29116409ace8855e64bf2e2b2d45871469