dolphin-emu-git build fails because new mGBA submodule is not initialized

This commit added mGBA as a submodule. Because of this we now need to initialize it or CMake fails.

Initialization should look something like this:

...
source=(
        "$pkgname::git+https://github.com/$_mainpkgname/$_projectname"
        "mGBA::git+https://github.com/mgba-emu/mgba.git")
sha256sums=(
        'SKIP'
        'SKIP'
)
...
prepare() {
        cd "$srcdir/$_sourcedirectory/"
        if [ -d 'build/' ]; then rm -rf 'build/'; fi
        mkdir 'build/'
        git submodule init
        git config submodule.mgba.url "$srcdir/mGBA"
        git submodule update
}
...

Happy to open a PR if you'd prefer