gallium: wrap u_screen in extern "C" for c++

Some drivers (notabily SWR) are written in C++, and as such they need
access to C headers with extern "C". So lets add that.

src/gallium/auxiliary/util/u_screen.h

@@ -24,6 +24,14 @@
 struct pipe_screen;
 enum pipe_cap;
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int
 u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
                                  enum pipe_cap param);
+
+#ifdef __cplusplus
+};
+#endif