headers: Sync up some header guard changes from drm-next.

This pulls in pieces of drm-next d65d31388a23
("Merge tag 'drm-misc-next-fixes-2017-11-07' of
git://anongit.freedesktop.org/drm/drm-misc into drm-next")

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/include/drm/vmwgfx_drm.h b/include/drm/vmwgfx_drm.h
index 5b68b4d..d325a41 100644
--- a/include/drm/vmwgfx_drm.h
+++ b/include/drm/vmwgfx_drm.h
@@ -30,6 +30,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 #define DRM_VMW_MAX_SURFACE_FACES 6
 #define DRM_VMW_MAX_MIP_LEVELS 24
 
@@ -1087,4 +1091,9 @@
 	enum drm_vmw_extended_context req;
 	struct drm_vmw_context_arg rep;
 };
+
+#if defined(__cplusplus)
+}
+#endif
+
 #endif