kernel: Add constructors to SharedMemory

kernel/src/memory/SharedMemory.h

@@ -14,6 +14,9 @@ struct SharedMemory
     SharedPtr<Device> device {};
     int refs { 0 };
 
+    SharedMemory() = default;
+    SharedMemory(SharedMemory&&) = default;
+
     static Result<u64> create(u8* mem, off_t offset, usize count);
 
     Result<void> grow_forward(u8* mem, usize count);
@@ -22,6 +25,8 @@ struct SharedMemory
     Result<void> map(u64 virt, int flags, off_t offset, usize count);
 
     void free();
+
+    ~SharedMemory() = default;
 };
 
 extern HashMap<u64, SharedMemory> g_shared_memory_map;