[macOS] Fix build errors in media_utils.cpp

[shinra-electric]

Changes AVCodec* to a constant rather than a variable in two places.

Building on macOS for Arm with Xcode 14.1 would fail with the following errors:

/Users/XXXX/Downloads/rpcs3/rpcs3/util/media_utils.cpp:702:17: error: cannot initialize a variable of type 'AVCodec *' with an rvalue of type 'const AVCodec *'
                        if (AVCodec* encoder = avcodec_find_encoder(used_codec); !!encoder)
                                     ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/XXXX/Downloads/rpcs3/rpcs3/util/media_utils.cpp:727:19: error: cannot initialize a variable of type 'AVCodec *' with an rvalue of type 'const AVCodec *'
                                        if (AVCodec* encoder = avcodec_find_encoder(codec->id); !!encoder)
                                                     ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Allows RPCS3 to build on macOS again

Testing:
1: Build on macOS (Edit: ARM and Intel)
2: Check that recording still works.

Should be checked by @Megamouse as they wrote the initial code.