MAINTAINERS: add entries for the key build bits

MAINTAINERS: add entries for the key build bits

Changes to the build files are a bit special in that they usually go
through other maintainer trees. However considering the build system
is the root of everything a developer is likely to do we should at
least set it out in MAINTAINERS.

I'm going to nominate Paolo for meson stuff given the conversion was
his passion project. I'm happy to cast an eye over configure stuff
considering a lot of the cross compile logic is in there anyway.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20221027183637.2772968-16-alex.bennee@linaro.org>