igvm: use idiomatic meson conditional for IGVM build files

Replace the explicit igvm.found() check with system_ss.add(when:,
if_true:), matching the pattern used by all other optional backends.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-ID: <20260626-microvm_device_tree-v6-2-9cd13cf057e2@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

backends/meson.build

diff --git a/backends/meson.build b/backends/meson.build
index 60021f4..aabfaea 100644
--- a/backends/meson.build
+++ b/backends/meson.build
@@ -34,11 +34,8 @@
 endif
 system_ss.add(when: gio, if_true: files('dbus-vmstate.c'))
 system_ss.add(when: 'CONFIG_SGX', if_true: files('hostmem-epc.c'))
-if igvm.found()
-  system_ss.add(igvm)
-  system_ss.add(files('igvm-cfg.c'), igvm)
-  system_ss.add(files('igvm.c'), igvm)
-endif
+
+system_ss.add(when: igvm, if_true: [files('igvm-cfg.c', 'igvm.c')])
 
 system_ss.add(when: 'CONFIG_SPDM_SOCKET', if_true: files('spdm-socket.c'))