[FASTCGI] const on fwrite pointer fix for fcgi_stdio.h

[Erik Aronesty]

for compatibility:

FCGI_fwrite(void*, size_t, size_t, FCGI_FILE*)

should be

FCGI_fwrite(const void*, size_t, size_t, FCGI_FILE*)

(i have to patch the header every time i install, not a big deal tho)

- erik