samples: cpp: hello_world: add cpp target tag

Add missing 'cpp' tag to ensure consistent sample tagging.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>

samples/cpp/hello_world/sample.yaml

@@ -2,7 +2,9 @@ sample:
   description: Hello World C++ sample, the simplest C++ Zephyr application
   name: hello cpp world
 common:
-  tags: introduction
+  tags:
+    - introduction
+    - cpp
   integration_platforms:
     - qemu_riscv32
   harness: console
@@ -22,4 +24,3 @@ tests:
       - intel_ish_5_4_1
       - intel_ish_5_6_0
       - intel_ish_5_8_0
-      - esp32c6_devkitc/esp32c6/lpcore