fs-util: include fcntl.h in fs-util.h

fs-util: include fcntl.h in fs-util.h

fs-util.h provides access_nofollow() but it did not include neither
fcntl.h nor unistd.h, which define F_OK and friends. Hence we cannot use
the function without including one of the headers. Let's include fcntl.h
in fs-util.h, then we can use the function by simply including fs-util.h.