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/tests/complex_test.cpp b/tests/complex_test.cpp
index 746c0be..a8f6777 100644
--- a/tests/complex_test.cpp
+++ b/tests/complex_test.cpp
@@ -23,6 +23,7 @@
 #define __INTRODUCED_IN(x)
 #define __INTRODUCED_IN_32(x)
 #define __INTRODUCED_IN_64(x)
+#define __INTRODUCED_IN_FUTURE
 #endif
 
 // libc++ actively gets in the way of including <complex.h> from C++, so we