Put back inline definitions if using an old API.

All these inlines were turned in to out of line definitions in L.
This brings us a step closer to being able to just use the current
bionic headers for the NDK, rather than having many old versions of
them.

Change-Id: Ie010bc727d78d3742abc577c70f6578db2e68625
diff --git a/libc/include/errno.h b/libc/include/errno.h
index 1a36b7a..82f4b42 100644
--- a/libc/include/errno.h
+++ b/libc/include/errno.h
@@ -46,6 +46,10 @@
 /* a macro expanding to the errno l-value */
 #define  errno   (*__errno())
 
+#if __ANDROID_API__ < 21
+#include <android/legacy_errno_inlines.h>
+#endif
+
 __END_DECLS
 
 #endif /* _ERRNO_H */