hw/audio/fmopl: Move ENV_CURVE to .heap to save 32KiB of .bss

This buffer is only used by the adlib audio device. Move it to
the .heap to release 32KiB of .bss (size reported on x86_64 host).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>