Fix build error for original theos framework.

Explicitly specify internal generator to fix below build error for original theos framework.

NOTE: Ryan Petrich forked theos has been modified default generator to internal from substrate.

````
/opt/theos/include/substrate.h:138:41: error: cast of an Objective-C pointer to 'char *' is disallowed with ARC
    void *pointer(ivar == NULL ? NULL : reinterpret_cast<char *>(self) + ivar_getOffset(ivar));
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
````