target/s390x: fix meson.build issue

files() is needed to avoid

../meson.build:977:2: ERROR: File tcg-stub.c does not exist.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/s390x/meson.build

@@ -21,7 +21,7 @@ s390x_ss.add(when: 'CONFIG_TCG', if_true: files(
   'vec_helper.c',
   'vec_int_helper.c',
   'vec_string_helper.c',
-), if_false: 'tcg-stub.c')
+), if_false: files('tcg-stub.c'))
 
 s390x_ss.add(when: 'CONFIG_KVM', if_true: files('kvm.c'), if_false: files('kvm-stub.c'))