Add missing <stdexcept> header for std::invalid_argument

[MarkLTZ]

In file included from crypto/equihash.cpp:20:
./crypto/equihash.h: In function ‘boolEhBasicSolve(unsignedint, unsignedint, const eh_HashState&, std::function<bool(std::vector<unsignedchar>)>, std::function<bool(EhSolverCancelCheck)>)’:
./crypto/equihash.h:235:20: error: ‘invalid_argument’ is not a member of ‘std’
235 | throw std::invalid_argument("Unsupported Equihash parameters");
| ^~~~~~~~~~~~~~~~
./crypto/equihash.h: In function ‘boolEhOptimisedSolve(unsignedint, unsignedint, const eh_HashState&, std::function<bool(std::vector<unsignedchar>)>, std::function<bool(EhSolverCancelCheck)>)’:
./crypto/equihash.h:261:20: error: ‘invalid_argument’ is not a member of ‘std’
261 | throw std::invalid_argument("Unsupported Equihash parameters");
| ^~~~~~~~~~~~~~~~
make[2]: *** [Makefile:3069: crypto/libbitcoin_crypto_a-equihash.o] Error 1

To compile on Fedora 32 with gcc version 10.2.1, needs to include <stdexcept>.