GCC 7.x warning cleanup - pointer conversion and unused variable

[kcgen]

jitter.c: In function ‘jitter_buffer_ctl’:
jitter.c:802:28: warning: ISO C forbids conversion of object pointer to function pointer type [-Wpedantic]
          jitter->destroy = (void (*) (void *))ptr;
                            ^
scal.c: In function ‘speex_decorrelate’:
scal.c:155:11: warning: unused variable ‘N’ [-Wunused-variable]
       int N=2*st->frame_size;
           ^

Feel free to build with -Wall -pedantic to help catch these.
Cheers.