make freebsd copy_file_range return type signed

Author: rpkak

lib/std/c/freebsd.zig

@@ -29,7 +29,7 @@ comptime {
 pub extern "c" fn ptrace(request: c_int, pid: pid_t, addr: caddr_t, data: c_int) c_int;
 
 pub extern "c" fn kinfo_getfile(pid: pid_t, cntp: *c_int) ?[*]kinfo_file;
-pub extern "c" fn copy_file_range(fd_in: fd_t, off_in: ?*off_t, fd_out: fd_t, off_out: ?*off_t, len: usize, flags: u32) usize;
+pub extern "c" fn copy_file_range(fd_in: fd_t, off_in: ?*off_t, fd_out: fd_t, off_out: ?*off_t, len: usize, flags: u32) isize;
 
 pub extern "c" fn sendfile(
     in_fd: fd_t,