Fix `-Wunused-function` warning when compiling with clang

[Gabriel de Quadros Ligneul]

When I compile a Titan program with clang, I get those warnings:

./foo.c:144:24: warning: unused function '_integer2str' [-Wunused-function]
inline static TString* _integer2str (lua_State *L, lua_Integer i) {
                       ^
./foo.c:150:24: warning: unused function '_float2str' [-Wunused-function]
inline static TString* _float2str (lua_State *L, lua_Number f) {
                       ^

These are related to the compiler internals and the end user shouldn't see them.