kernel: Correct class/struct mixups.

Almost certainly harmless. Spotted by Clang.

headers/private/kernel/vm/VMArea.h

@@ -148,9 +148,9 @@ protected:
 			status_t			Init(const char* name, uint32 allocationFlags);
 
 protected:
-			friend class VMAddressSpace;
-			friend class VMKernelAddressSpace;
-			friend class VMUserAddressSpace;
+			friend struct VMAddressSpace;
+			friend struct VMKernelAddressSpace;
+			friend struct VMUserAddressSpace;
 
 protected:
 			void				SetBase(addr_t base)	{ fBase = base; }

headers/private/kernel/vm/VMCache.h

@@ -20,7 +20,7 @@
 
 
 struct kernel_args;
-class ObjectCache;
+struct ObjectCache;
 
 
 enum {