Revert "frameworks/base: libexpat now exports its headers."

This reverts commit 2f31b66c04b1e80fd039418edf41e74f67b68143.

Change-Id: Ib4ce610d1767db5c72c54d33520e8bdf50171a18
diff --git a/tools/aapt/StringPool.h b/tools/aapt/StringPool.h
index be843a8..dbe8c85 100644
--- a/tools/aapt/StringPool.h
+++ b/tools/aapt/StringPool.h
@@ -20,7 +20,7 @@
 #include <ctype.h>
 #include <errno.h>
 
-#include <expat.h>
+#include <libexpat/expat.h>
 
 using namespace android;