automake: Include virtgpu_drm.h in the release tarball

The plan is to use this version of virtgpu_drm.h in mesa and drop mesa's
local copy.
To actually use this header it needs to be shipped in the tarball.

This was missed in c745e541a9d8dfd3fb5e1ac57297e58d34d9328f

Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/Makefile.sources b/Makefile.sources
index 1a1f0fe..a57036a 100644
--- a/Makefile.sources
+++ b/Makefile.sources
@@ -33,7 +33,8 @@
 	include/drm/sis_drm.h \
 	include/drm/tegra_drm.h \
 	include/drm/vc4_drm.h \
-	include/drm/via_drm.h
+	include/drm/via_drm.h \
+	include/drm/virtgpu_drm.h
 
 LIBDRM_INCLUDE_VMWGFX_H_FILES := \
 	include/drm/vmwgfx_drm.h