Update header versions for NDK platform fixes.

Bug: http://b/28178111
Change-Id: Icd638673b409aa43a91490f77c6b4d79c9ea20d9
diff --git a/libc/include/sys/auxv.h b/libc/include/sys/auxv.h
index 4611fcc..2fa637e 100644
--- a/libc/include/sys/auxv.h
+++ b/libc/include/sys/auxv.h
@@ -33,7 +33,7 @@
 
 __BEGIN_DECLS
 
-unsigned long int getauxval(unsigned long int type) __INTRODUCED_IN(21);
+unsigned long int getauxval(unsigned long int type) __INTRODUCED_IN(18);
 
 __END_DECLS