Guard fortify inlines with __ANDROID_API__.

Test: make checkbuild with libc ndk_library patches applied
Change-Id: Ic5e248994c4f2702b0f4d6dfeda787187ea86017
diff --git a/libc/include/poll.h b/libc/include/poll.h
index d75c2d0..0fa55d2 100644
--- a/libc/include/poll.h
+++ b/libc/include/poll.h
@@ -53,6 +53,7 @@
 
 #if defined(__BIONIC_FORTIFY)
 
+#if __ANDROID_API__ >= 23
 __BIONIC_FORTIFY_INLINE
 int poll(struct pollfd* fds, nfds_t fd_count, int timeout) {
 #if defined(__clang__)
@@ -84,6 +85,7 @@
   return __ppoll_real(fds, fd_count, timeout, mask);
 #endif
 }
+#endif /* __ANDROID_API__ >= 23 */
 
 #endif