testsuite: Make ztress header C++ compatible

This allows ztress to be linked correctly from C++ based tests.

Signed-off-by: Benjamin Gwin <bgwin@google.com>

subsys/testsuite/ztest/include/ztress.h

@@ -9,6 +9,10 @@
 #include <zephyr/sys/util.h>
 #include <zephyr/kernel.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** @internal Internal ID's to distinguish context type. */
 #define ZTRESS_ID_THREAD 0
 #define ZTRESS_ID_K_TIMER 1
@@ -229,4 +233,8 @@ int ztress_preempt_count(uint32_t id);
  */
 uint32_t ztress_optimized_ticks(uint32_t id);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* TESTSUITE_ZTEST_INCLUDE_ZTRESS_H__ */