[nrf fromtree] cmake: use the warnings_as_errors flag for cpp files

C++ file compilation is actually missing the warning-as-error handling,
causing warnings in build files to be unnoticed in CI. Add a flag to
handle them as well.

Signed-off-by: Fabio Baltieri <[email protected]>
(cherry picked from commit 56dcafe)

Author: fabiobaltieri

CMakeLists.txt

@@ -152,6 +152,7 @@ zephyr_compile_options($<$<COMPILE_LANGUAGE:CXX>:$<TARGET_PROPERTY:compiler-cpp,
 # Extra warnings options for twister run
 if (CONFIG_COMPILER_WARNINGS_AS_ERRORS)
   zephyr_compile_options($<$<COMPILE_LANGUAGE:C>:$<TARGET_PROPERTY:compiler,warnings_as_errors>>)
+  zephyr_compile_options($<$<COMPILE_LANGUAGE:CXX>:$<TARGET_PROPERTY:compiler,warnings_as_errors>>)
   zephyr_compile_options($<$<COMPILE_LANGUAGE:ASM>:$<TARGET_PROPERTY:asm,warnings_as_errors>>)
   zephyr_link_libraries($<TARGET_PROPERTY:linker,warnings_as_errors>)
 endif()