Add __INTRODUCED_IN_FUTURE for unreleased APIs.

Future API levels aren't known (e.g. 25 could be a maintenance release
of N that doesn't contain any bionic updates), so use a placeholder
macro that we can find and replace with the actual API level before each
release.

Bug: http://b/28178111
Change-Id: I667fe53ea1ac49b64135170fc30d5dbe9df94e29
diff --git a/libc/include/grp.h b/libc/include/grp.h
index 9b4a518..42cf223 100644
--- a/libc/include/grp.h
+++ b/libc/include/grp.h
@@ -55,10 +55,10 @@
 struct group* getgrnam(const char *);
 #if __POSIX_VISIBLE >= 200112 || __XPG_VISIBLE
 /* Android has thousands and thousands of ids to iterate through */
-struct group* getgrent(void) __attribute__((warning("getgrent is inefficient on Android")))
-__INTRODUCED_IN(25);
-void setgrent(void) __INTRODUCED_IN(25);
-void endgrent(void) __INTRODUCED_IN(25);
+struct group* getgrent(void)
+  __attribute__((warning("getgrent is inefficient on Android"))) __INTRODUCED_IN_FUTURE;
+void setgrent(void) __INTRODUCED_IN_FUTURE;
+void endgrent(void) __INTRODUCED_IN_FUTURE;
 int getgrgid_r(gid_t, struct group*, char*, size_t, struct group**) __INTRODUCED_IN(24);
 int getgrnam_r(const char*, struct group*, char*, size_t, struct group**) __INTRODUCED_IN(24);
 #endif