newly added string_split function in common.h generates warnings

[raulod]

I recommend adding __attribute__((unused)) attribute or relocate function from common.h

~/llama.cpp/common/common.h:412:26: warning: ‘std::vector<T> string_split(const string&, char) 
[with T = std::__cxx11::basic_string<char>]’ defined but not used [-Wunused-function]
 std::vector<std::string> string_split<std::string>(const std::string & input, char separator)