libobs/audio-monitoring: Fix compile error

Created by: norihiro

Description

Add a missing ;.

Motivation and Context

To build this branch on Linux.

How Has This Been Tested?

Compiled on Fedora 35 with these options.

cmake -DUNIX_STRUCTURE=1 -DBUILD_BROWSER=OFF -D ENABLE_PIPEWIRE=ON -D BUILD_VST=OFF -D ENABLE_LIBFDK=ON -D OBS_VERSION_OVERRIDE=27.2.5 -D ENABLE_AJA=OFF -D USE_NEW_MPEGTS_OUTPUT=OFF

Types of changes

• Bug fix (non-breaking change which fixes an issue)