Only rename sendfile to sendfile64 if we're L or newer.

Without this, setting __FILE_OFFSET_BITS to 64 and targeting pre-L
made sendfile entirely unavailable.

Bug: https://github.com/android-ndk/ndk/issues/333
Test: builds
Change-Id: I82d326ff244473dd5443632ff6c19d726d5e29ee
diff --git a/libc/include/sys/sendfile.h b/libc/include/sys/sendfile.h
index 3ac8fdf..dccdec5 100644
--- a/libc/include/sys/sendfile.h
+++ b/libc/include/sys/sendfile.h
@@ -34,7 +34,7 @@
 
 __BEGIN_DECLS
 
-#if defined(__USE_FILE_OFFSET64)
+#if defined(__USE_FILE_OFFSET64) && __ANDROID_API__ >= __ANDROID_API_L__
 ssize_t sendfile(int out_fd, int in_fd, off_t* offset, size_t count) __RENAME(sendfile64)
   __INTRODUCED_IN(21);
 #else