funopen: Replace off64_t with off_t in funopen_seek
AC_SYS_LARGEFILE in configure.ac is setting needed defines to make 64bit off_t on relevant platforms.
Fixes build on musl
| ../../../../../../../../workspace/sources/libbsd/src/funopen.c:68:28: error: unknown type name 'off64_t'; did you mean 'off_t'? | funopen_seek(void *cookie, off64_t *offset, int whence) | ^~~~~~~ | off_t
Signed-off-by: Khem Raj raj.khem@gmail.com