[mutter] build: Include missing file

[Florian Müllner <fmuellner src gnome org>]

commit 2d8eaa082f1774585b29492f7b4479ce422d4785
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Mar 14 02:59:53 2017 +0100

    build: Include missing file

 src/Makefile.am |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/Makefile.am b/src/Makefile.am
index 6d9b5e2..17354ea 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -91,6 +91,7 @@ libmutter_@LIBMUTTER_API_VERSION@_la_SOURCES =        \
        backends/meta-cursor-tracker-private.h  \
        backends/meta-cursor-renderer.c         \
        backends/meta-cursor-renderer.h         \
+       backends/meta-dnd-private.h             \
        backends/meta-egl.c                     \
        backends/meta-egl.h                     \
        backends/meta-egl-ext.h                 \