Don't redefine __ANDROID_API__.

Bug: http://b/28178111
Change-Id: I00ded8f90694d9ed28f3b4c97778635f5a564592
diff --git a/libc/include/android/api-level.h b/libc/include/android/api-level.h
index 2d2f096..b230cef 100644
--- a/libc/include/android/api-level.h
+++ b/libc/include/android/api-level.h
@@ -33,6 +33,8 @@
  * Magic version number for a current development build, which has
  * not yet turned into an official release.
  */
+#ifndef __ANDROID_API__
 #define __ANDROID_API__ 10000
+#endif
 
 #endif /* ANDROID_API_LEVEL_H */