avdevice/avfoundation: add external video devices

Video devices categorized by AVFoundation as
'AVCaptureDeviceTypeExternal(Unknown)' (like USB video streams) were not
recognized by libavdevice.

Signed-off-by: Theo Fabi <fabi.theo@gmail.com>

libavdevice/avfoundation.m

@@ -788,6 +788,9 @@ static NSArray* getDevicesWithMediaType(AVMediaType mediaType) {
         #endif
         #if (TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MIN_REQUIRED >= 170000 || (TARGET_OS_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED >= 140000))
             [deviceTypes addObject: AVCaptureDeviceTypeContinuityCamera];
+            [deviceTypes addObject: AVCaptureDeviceTypeExternal];
+        #elif (TARGET_OS_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED < 140000)
+            [deviceTypes addObject: AVCaptureDeviceTypeExternalUnknown];
         #endif
     } else if (mediaType == AVMediaTypeAudio) {
         #if (TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MIN_REQUIRED >= 170000 || (TARGET_OS_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED >= 140000))