meson: drop parens when appending to list

Meson does not care either way, so let's use the simpler syntax. And files()
already gives a list, so nesting this in a list wouldn't be necessary even
if meson did not flatten everything.

src/boot/efi/meson.build

@@ -147,10 +147,10 @@ if have_gnu_efi
                                                  + compile_args,
                                        depend_files : efi_headers)
                 if (common_sources + systemd_boot_sources).contains(file)
-                        systemd_boot_objects += [o_file]
+                        systemd_boot_objects += o_file
                 endif
                 if (common_sources + stub_sources).contains(file)
-                        stub_objects += [o_file]
+                        stub_objects += o_file
                 endif
         endforeach