Added includes for <string.h> to explicitly declare 'memcpy' where needed.
This message is typical of messages suppressed by declaring 'memcpy':
warning: In file included from depend/secp256k1-zkp/src/secp256k1.c:14:0:
warning: depend/secp256k1-zkp/src/ecmult_impl.h: In function ‘secp256k1_ecmult_context_clone’:
warning: depend/secp256k1-zkp/src/ecmult_impl.h:186:9: warning: implicit declaration of function ‘memcpy’ [-Wimplicit-function-declaration]
warning: memcpy(dst->pre_g, src->pre_g, size);
warning: ^~~~~~
