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/shared/matrix.h b/shared/matrix.h
index 47354f6..e5cf636 100644
--- a/shared/matrix.h
+++ b/shared/matrix.h
@@ -24,6 +24,10 @@
 #ifndef WESTON_MATRIX_H
 #define WESTON_MATRIX_H
 
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
 enum weston_matrix_transform_type {
 	WESTON_MATRIX_TRANSFORM_TRANSLATE	= (1 << 0),
 	WESTON_MATRIX_TRANSFORM_SCALE		= (1 << 1),
@@ -71,4 +75,8 @@
 #  define MATRIX_TEST_EXPORT static
 #endif
 
+#ifdef  __cplusplus
+}
+#endif
+
 #endif /* WESTON_MATRIX_H */