fix minor namespace issue in unistd.h

the F_* macros associated with the lockf function are XSI-shaded (like
the lockf function itself) and should only be exposed when the
function is.

include/unistd.h

@@ -128,12 +128,11 @@ long fpathconf(int, int);
 long sysconf(int);
 size_t confstr(int, char *, size_t);
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 #define F_ULOCK 0
 #define F_LOCK  1
 #define F_TLOCK 2
 #define F_TEST  3
-
-#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 int setreuid(uid_t, uid_t);
 int setregid(gid_t, gid_t);
 int lockf(int, int, off_t);