games/ponscripter-07th-mod: unbreak build with clang 16

bstrwrap.cpp:724:1: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
register int i, l;
^~~~~~~~~
bstrwrap.cpp:724:1: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
register int i, l;
^~~~~~~~~
bstrwrap.cpp:725:1: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
register unsigned char cx;
^~~~~~~~~
bstrwrap.cpp:726:1: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
register unsigned char * pdata;
^~~~~~~~~

Reported by: pkg-fallout