Add missing "extern" to fix building with -fno-common

Clang 11 has switched to default to -fno-common, just like GCC 10
has. This requires properly declaring varibles with extern in headers.

Rebased: Some parts of the original commit applied to bits that don't
exist in the 3.0 branch.

(cherry picked from commit 300439c055)

modules/codec/dmo/dmo.h

@@ -21,21 +21,21 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  *****************************************************************************/
 
-const GUID IID_IUnknown;
-const GUID IID_IClassFactory;
-const GUID IID_IWMCodecPrivateData;
-const GUID IID_IMediaObject;
-const GUID IID_IMediaBuffer;
-const GUID MEDIATYPE_Video;
-const GUID MEDIATYPE_Audio;
-const GUID MEDIASUBTYPE_PCM;
-const GUID FORMAT_VideoInfo;
-const GUID FORMAT_WaveFormatEx;
-const GUID GUID_NULL;
-const GUID MEDIASUBTYPE_I420;
-const GUID MEDIASUBTYPE_YV12;
-const GUID MEDIASUBTYPE_RGB24;
-const GUID MEDIASUBTYPE_RGB565;
+extern const GUID IID_IUnknown;
+extern const GUID IID_IClassFactory;
+extern const GUID IID_IWMCodecPrivateData;
+extern const GUID IID_IMediaObject;
+extern const GUID IID_IMediaBuffer;
+extern const GUID MEDIATYPE_Video;
+extern const GUID MEDIATYPE_Audio;
+extern const GUID MEDIASUBTYPE_PCM;
+extern const GUID FORMAT_VideoInfo;
+extern const GUID FORMAT_WaveFormatEx;
+extern const GUID GUID_NULL;
+extern const GUID MEDIASUBTYPE_I420;
+extern const GUID MEDIASUBTYPE_YV12;
+extern const GUID MEDIASUBTYPE_RGB24;
+extern const GUID MEDIASUBTYPE_RGB565;
 
 
 #ifndef _WIN32