hw/nvram: Restrict fw_cfg QOM interface to sysemu and tools

fw_cfg QOM interface is required by system emulation and
qemu-storage-daemon. User-mode emulation doesn't need it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220111184309.28637-3-f4bug@amsat.org>

hw/nvram/meson.build

-# QOM interfaces must be available anytime QOM is used.
-qom_ss.add(files('fw_cfg-interface.c'))
+if have_system or have_tools
+  # QOM interfaces must be available anytime QOM is used.
+  qom_ss.add(files('fw_cfg-interface.c'))
+endif
 
 softmmu_ss.add(files('fw_cfg.c'))
 softmmu_ss.add(when: 'CONFIG_CHRP_NVRAM', if_true: files('chrp_nvram.c'))