meson: build contrib/plugins with meson

Tried to unify this meson.build with tests/tcg/plugins/meson.build but
the resulting modules are not output in the right directory.

Originally proposed by Anton Kochkov, thank you!

Solves: https://gitlab.com/qemu-project/qemu/-/issues/1710
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20241023212812.1376972-3-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

meson.build

@@ -3690,6 +3690,10 @@ subdir('accel')
 subdir('plugins')
 subdir('ebpf')
 
+if 'CONFIG_TCG' in config_all_accel
+  subdir('contrib/plugins')
+endif
+
 common_user_inc = []
 
 subdir('common-user')