Use st_mtime instead of st_mtim.tv_sec

The latter might be linux specific, and as we don't need nanosecond
precision we can just use st_mtime.

c/ton/main.c

	}
	if (last_modified != NULL) {
		*last_modified = f_stat.st_mtim.tv_sec;
		*last_modified = f_stat.st_mtime;
	}
	char *buf = malloc(f_stat.st_size + 1);