xen/pt: fix syntax error that causes FTBFS in some configurations

When Qemu is built with --enable-xen and --disable-xen-pci-passthrough
and the target os is linux, the build fails with:

meson.build:3477:2: ERROR: File xen_pt_stub.c does not exist.

Fixes: 582ea95f5f93 ("meson: convert hw/xen")

Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <5f1342a13c09af77b1a7b0aeaba5955bcea89731.1667242033.git.brchuckz@aol.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/xen/meson.build

@@ -18,7 +18,7 @@ if have_xen_pci_passthrough
     'xen_pt_msi.c',
   ))
 else
-  xen_specific_ss.add('xen_pt_stub.c')
+  xen_specific_ss.add(files('xen_pt_stub.c'))
 endif
 
 specific_ss.add_all(when: ['CONFIG_XEN', xen], if_true: xen_specific_ss)