obs-libfdk: Enable 7.1 channel surround for Linux

The surround audio was introduced by bbac3280c1 but 7.1 channel audio
was disabled since libfdk-aac >= 0.1.3 is required.
Now the minimum Ubuntu version is 18.04, which provide libfdk-aac 0.1.5.

plugins/obs-libfdk/obs-libfdk.c

@@ -132,14 +132,9 @@ static void *libfdk_create(obs_data_t *settings, obs_encoder_t *encoder)
 		mode = MODE_1_2_2_1;
 		break;
 
-		/* lib_fdk-aac > 1.3 required for 7.1 surround;
-         * uncomment if available on linux build
-         */
-#ifndef __linux__
 	case 8:
 		mode = MODE_7_1_REAR_SURROUND;
 		break;
-#endif
 
 	default:
 		blog(LOG_ERROR, "Invalid channel count");