disas: avoid including everything in headers compiled from C++

disas/arm-a64.cc is careful to include only the bare minimum that
it needs---qemu/osdep.h and disas/bfd.h.  Unfortunately, disas/bfd.h
then includes qemu-common.h, which brings in qemu/option.h and from
there we get the kitchen sink.

This causes problems because for example QEMU's atomic macros
conflict with C++ atomic types.  But really all that bfd.h needs
is the fprintf_function typedef, so replace the inclusion of
qemu-common.h with qemu/fprintf-fn.h.

Reported-by: Sean Bruno <sbruno@freebsd.org>
Tested-by: Sean Bruno <sbruno@freebsd.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>