FileDescriptor: add operator=() so that the fcntl(F_DUPFD syscall compiles :)

kernel/include/fs/FileDescriptor.h

@@ -43,6 +43,8 @@ struct Descriptor
     Descriptor(const Descriptor& other);
     Descriptor();
 
+    const Descriptor& operator=(const Descriptor& other);
+
   private:
     bool m_is_open;
     bool m_can_read;

kernel/src/fs/FileDescriptor.cpp

@@ -41,3 +41,13 @@ int Descriptor::seek(long offset)
     m_offset = (uint64_t)offset;
     return 0;
 }
+
+const Descriptor& Descriptor::operator=(const Descriptor& other)
+{
+    m_is_open = other.m_is_open;
+    m_can_read = other.m_can_read;
+    m_can_write = other.m_can_write;
+    m_offset = other.m_offset;
+    m_node = other.m_node;
+    return other;
+}