GCC9 build error

[gumb0]

Building with GCC 9 fails, see ethereum/aleth#5798 for some related fixes

[ 46%] Building CXX object libdevcore/CMakeFiles/devcore.dir/SHA3.cpp.o
In file included from /home/andrei/dev/retesteth/libdevcore/SHA3.cpp:22:
/home/andrei/dev/retesteth/libdevcore/SHA3.h: In function ‘dev::SecureFixedHash<32> dev::sha3Secure(dev::bytesConstRef)’:
/home/andrei/dev/retesteth/libdevcore/SHA3.h:41:131: error: implicitly-declared ‘constexpr dev::SecureFixedHash<32>::SecureFixedHash(const dev::SecureFixedHash<32>&)’ is deprecated [-Werror=deprecated-copy]
   41 | inline SecureFixedHash<32> sha3Secure(bytesConstRef _input) { SecureFixedHash<32> ret; sha3(_input, ret.writable().ref()); return ret; }
      |                                                                                                                                   ^~~
In file included from /home/andrei/dev/retesteth/libdevcore/SHA3.h:27,
                 from /home/andrei/dev/retesteth/libdevcore/SHA3.cpp:22:
/home/andrei/dev/retesteth/libdevcore/FixedHash.h:245:22: note: because ‘dev::SecureFixedHash<32>’ has user-provided ‘dev::SecureFixedHash<T>& dev::SecureFixedHash<T>::operator=(const dev::SecureFixedHash<T>&) [with unsigned int T = 32]’
  245 |  SecureFixedHash<T>& operator=(SecureFixedHash<T> const& _c)
      |                      ^~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [libdevcore/CMakeFiles/devcore.dir/build.make:167: libdevcore/CMakeFiles/devcore.dir/SHA3.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:239: libdevcore/CMakeFiles/devcore.dir/all] Error 2
make: *** [Makefile:84: all] Error 2

[winsvega]

fixed
6d528e4
65a80d0