tests: lib: cpp: add __maybe_unused to unused variable

The foos variable is not really used. Add __maybe_unused to silence
compiler.

Signed-off-by: Dawid Niedzwiecki <[email protected]>

Author: niedzwiecki-dawid

tests/lib/cpp/cxx/src/main.cpp

@@ -99,7 +99,7 @@ struct foo {
 /* Check that BUILD_ASSERT compiles. */
 BUILD_ASSERT(sizeof(foo) == sizeof(int));
 
-static struct foo foos[5];
+__maybe_unused static struct foo foos[5];
 /* Check that ARRAY_SIZE compiles. */
 BUILD_ASSERT(ARRAY_SIZE(foos) == 5, "expected 5 elements");