meson: test with c++26 too

The docs say that support is experimental, but it's better for us if we know
about any problems early.

Header tests pass without any issue with gcc-14.0.1-0.8.fc40.x86_64.

src/systemd/meson.build

@@ -79,6 +79,7 @@ if cxx_cmd != ''
                        '-std=c++17',
                        '-std=c++20',
                        '-std=c++23',
+                       '-std=c++26',
                       ]
                 if cxx.has_argument(opt)
                         opts += [['c++', opt]]