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/sys/cdefs.h b/libc/include/sys/cdefs.h
index 24b341e..3fcf163 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -459,6 +459,8 @@
 #define __DEPRECATED_IN(api_level) __AVAILABILITY(deprecated=api_level)
 #define __REMOVED_IN(api_level) __AVAILABILITY(obsoleted=api_level)
 
+#define __INTRODUCED_IN_FUTURE __INTRODUCED_IN(10000)
+
 #if __LP64__
 #define __INTRODUCED_IN_32(api_level)
 #define __INTRODUCED_IN_64 __INTRODUCED_IN