disable jpeg support in libyuv

Without this the ndk-build fails with:
Android NDK: Module vpxJNI depends on undefined modules: jpeg
diff --git a/JNI/Android.mk b/JNI/Android.mk
index 184f962..84287fc 100644
--- a/JNI/Android.mk
+++ b/JNI/Android.mk
@@ -1,6 +1,7 @@
 # Build libyuv.a
 include $(CLEAR_VARS)
 
+LIBYUV_DISABLE_JPEG := "yes"
 include $(BINDINGS_DIR)/libyuv/Android.mk
 
 # Build libwebm.a