Keep os/android-os.h compiling now we have __key in <sys/ipc.h>.

When we've switched the NDK to unified headers (and perhaps the
compatibility library too) we should send a whole new cleaned up
version of this file upstream...

Bug: N/A
Test: builds
Change-Id: Icf8cbd678b05bbb66bd1a4091a338c696d749478
diff --git a/os/os-android.h b/os/os-android.h
index c002509..96ff5ba 100644
--- a/os/os-android.h
+++ b/os/os-android.h
@@ -62,6 +62,7 @@
 #define SHM_HUGETLB    04000
 
 #define shmid_ds shmid64_ds
+#undef __key
 
 #include <stdio.h>
 #include <linux/ashmem.h>