symbol.cpp: shut up useless warnings

symbol.cpp: shut up useless warnings

../../sw/ginac/ginac/symbol.h: In constructor 'GiNaC::symbol::symbol()':
../../sw/ginac/ginac/symbol.h:112: warning: 'GiNaC::symbol::ret_type_tinfo' will be initialized after
../../sw/ginac/ginac/symbol.h:110: warning:   'unsigned int GiNaC::symbol::domain'
../../sw/ginac/ginac/symbol.cpp:47: warning:   when initialized here
../../sw/ginac/ginac/symbol.h: In constructor 'GiNaC::symbol::symbol(const std::string&, unsigned int)':
../../sw/ginac/ginac/symbol.h:112: warning: 'GiNaC::symbol::ret_type_tinfo' will be initialized after
../../sw/ginac/ginac/symbol.h:110: warning:   'unsigned int GiNaC::symbol::domain'
../../sw/ginac/ginac/symbol.cpp:75: warning:   when initialized here
../../sw/ginac/ginac/symbol.h: In constructor 'GiNaC::symbol::symbol(const std::string&, unsigned int, const void*, unsigned int)':

The warnings are a bit silly, but they can hide more useful ones.