Revert "dshowsrcwrapper: remove unused extern C"

This reverts commit d3c88d637d.

Reverting this since the entry point isn't found because of
C++ build.

sys/dshowsrcwrapper/gstdshowsrcwrapper.cpp

@@ -38,8 +38,12 @@ plugin_init (GstPlugin * plugin)
   return TRUE;
 }
 
+extern "C" {
+
 GST_PLUGIN_DEFINE (GST_VERSION_MAJOR,
     GST_VERSION_MINOR,
     "dshowsrcwrapper",
     "DirectShow sources wrapper plugin",
     plugin_init, VERSION, "LGPL", GST_PACKAGE_NAME, GST_PACKAGE_ORIGIN)
+
+}