CLEANUP: Add missing header to hlua_fcn.c

CLEANUP: Add missing header to hlua_fcn.c

Found with -Wmissing-prototypes:

    src/hlua_fcn.c:53:5: fatal error: no previous prototype for function 'hlua_checkboolean' [-Wmissing-prototypes]
    int hlua_checkboolean(lua_State *L, int index)
        ^
    src/hlua_fcn.c:53:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    int hlua_checkboolean(lua_State *L, int index)
    ^
    static
    1 error generated.