build: add Wreturn-type to Werror flags
build: add Wreturn-type to Werror flags

This is supported by GCC and Clang.
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
https://clang.llvm.org/docs/DiagnosticsReference.html#wreturn-type

Former-commit-id: 3836af1baebc00a9b48f21108942e584957e87b9
syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin2