add missing include directory

Should have been in change Id98e0870 (though it builds on my machine
even without this...)
diff --git a/tools/applypatch/Android.mk b/tools/applypatch/Android.mk
index bb1519e..d20d6c8 100644
--- a/tools/applypatch/Android.mk
+++ b/tools/applypatch/Android.mk
@@ -51,7 +51,7 @@
 LOCAL_MODULE := imgdiff
 LOCAL_FORCE_STATIC_EXECUTABLE := true
 LOCAL_MODULE_TAGS := eng
-LOCAL_C_INCLUDES += external/zlib
+LOCAL_C_INCLUDES += external/zlib external/bzip2
 LOCAL_STATIC_LIBRARIES += libz libbz
 
 include $(BUILD_HOST_EXECUTABLE)