Fix compile warning

Use long instead of size_t.

gbuf.c: In function 'gbuf_add_ch':
gbuf.c:36:2: warning: passing argument 2 of 'u_set_char_raw' from incompatible pointer type [enabled by default]
In file included from gbuf.c:7:0:
uchar.h:39:6: note: expected 'long int *' but argument is of type 'size_t *'
Remove obsolete comment