interface/cpp.cc: move cpp_generator::print_method_decl before first use
interface/cpp.cc: move cpp_generator::print_method_decl before first use

This will make it easier to convert this method
into a specialized method template in an upcoming commit.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl