arm_test_common: Add missing header guard

src/tests/core/arm/arm_test_common.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <tuple>
 #include <unordered_map>
 #include <vector>