gcc got a little stricter with argument types

[NixM0nk3y]

Allow compilation on more modern gcc. Tested and working on raspian - sorry I don't have access to older gcc to test backward compatibleness

[razed11]

Seems to have fixed this problem that I'm having on MacOS:

flac/encoder_wrap.c:3116:19: note: forward declaration of 'struct FLAC__StreamEncoder'
SWIGINTERN struct FLAC__StreamEncoder *new_FLAC__StreamEncoder(void){
                  ^
flac/encoder_wrap.c:4090:19: error: incomplete definition of type 'struct FLAC__StreamEncoder'
  if (arg1) (arg1)->private_ = arg2;