[3.2] disable unused-function warning in secp256k1

[linh2931]

Resolve #19

secp256k1 produced over 190 warnings in the form of

/home/lh/work/leap-main/libraries/libfc/secp256k1/secp256k1/src/field.h:44:13: warning: ‘secp256k1_fe_normalize’   declared ‘static’ but never defined [-Wunused-function]
   44 | static void secp256k1_fe_normalize(secp256k1_fe *r);
      |             ^~~~~~~~~~~~~~~~~~~~~~
/home/lh/work/leap-main/libraries/libfc/secp256k1/secp256k1/src/field.h:47:13: warning:                            ‘secp256k1_fe_normalize_weak’ declared ‘static’ but never defined [-Wunused-function]
   47 | static void secp256k1_fe_normalize_weak(secp256k1_fe *r);
.../home/lh/work/leap-main/libraries/libfc/secp256k1/secp256k1/src/field.h:44:13: warning: ‘secp256k1_fe_normalize’   declared ‘static’ but never defined [-Wunused-function]
   44 | static void secp256k1_fe_normalize(secp256k1_fe *r);
      |             ^~~~~~~~~~~~~~~~~~~~~~
/home/lh/work/leap-main/libraries/libfc/secp256k1/secp256k1/src/field.h:47:13: warning:                            ‘secp256k1_fe_normalize_weak’ declared ‘static’ but never defined [-Wunused-function]
   47 | static void secp256k1_fe_normalize_weak(secp256k1_fe *r);
...

As we consider secp256k1 as a system library and use it without any modifications, it is OK to disable the warning to avoid cluttering compile log.

Before the fix, Leap build log was 1,710 lines long; after the fix, it is 1,096 lines. Only 22 warnings remain. We can hopefully have a clean build in release/3.2.

[spoonincode]

looks good