u_threaded_context: Support including from C++

Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed By: Bill Kristiansen <billkris@microsoft.com>

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13670>

src/gallium/auxiliary/util/u_threaded_context.h

@@ -202,6 +202,10 @@
 #include "util/u_thread.h"
 #include "util/slab.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct threaded_context;
 struct tc_unflushed_batch_token;
 
@@ -608,4 +612,8 @@ tc_buffer_disable_cpu_storage(struct pipe_resource *buf)
    }
 }
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif