meson.build: Always require an objc compiler on macos hosts

meson.build: Always require an objc compiler on macos hosts

We currently only insist that an ObjectiveC compiler is present on
macos hosts if we're building the Cocoa UI.  However, since then
we've added some other parts of QEMU which are also written in ObjC:
the coreaudio audio backend, and the vmnet net backend.  This means
that if you try to configure QEMU on macos with --disable-cocoa the
build will fail:

../meson.build:3741:13: ERROR: No host machine compiler for 'audio/coreaudio.m'

Since in practice any macos host will have an ObjC compiler
available, rather than trying to gate the compiler detection on an
increasingly complicated list of every bit of QEMU that uses ObjC,
just require it unconditionally on macos hosts.

Resolves https://gitlab.com/qemu-project/qemu/-/issues/2138

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240311133334.3991537-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>