scripts/coverity-scan/COMPONENTS.md: Add a 'plugins' category

Cover the TCG plugins files under their own Coverity category.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250811094341.91597-1-philmd@linaro.org>

scripts/coverity-scan/COMPONENTS.md

@@ -147,6 +147,9 @@ tcg
 system
   ~ .*/qemu(/system/.*|/accel/.*)
 
+plugins
+  ~ .*/qemu(/contrib|/tests/tcg)?/plugins/.*
+
 (headers)
   ~ .*/qemu(/include/.*)