arm_exclusive_monitor: Properly qualify friend class

* Otherwise we get compile errors on compilers that are not MSVC

src/core/arm/dynarmic/arm_exclusive_monitor.h

@@ -32,7 +32,7 @@ public:
     bool ExclusiveWrite64(std::size_t core_index, VAddr vaddr, u64 value) override;
 
 private:
-    friend class ARM_Dynarmic;
+    friend class ::ARM_Dynarmic;
     Dynarmic::ExclusiveMonitor monitor;
     Memory::MemorySystem& memory;
 };