sdk: be C++ friendly

This renames the weston_surface's private member to configure_private
and externs "C" the headers of the SDK.
diff --git a/src/compositor.h b/src/compositor.h
index dc03aeb..0ad60ef 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -24,6 +24,10 @@
 #ifndef _WAYLAND_SYSTEM_COMPOSITOR_H_
 #define _WAYLAND_SYSTEM_COMPOSITOR_H_
 
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
 #include <pixman.h>
 #include <xkbcommon/xkbcommon.h>
 #include <wayland-server.h>
@@ -496,7 +500,7 @@
 	 * are the sx and sy paramerters supplied to surface::attach .
 	 */
 	void (*configure)(struct weston_surface *es, int32_t sx, int32_t sy, int32_t width, int32_t height);
-	void *private;
+	void *configure_private;
 };
 
 enum weston_key_state_update {
@@ -880,4 +884,8 @@
 			enum wl_output_transform transform,
 			pixman_box32_t rect);
 
+#ifdef  __cplusplus
+}
+#endif
+
 #endif