Kernel: Use default con/de-structors

This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40

With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.

Kernel/Net/EthernetFrameHeader.h

@@ -33,8 +33,8 @@
 
 class [[gnu::packed]] EthernetFrameHeader {
 public:
-    EthernetFrameHeader() { }
-    ~EthernetFrameHeader() { }
+    EthernetFrameHeader() = default;
+    ~EthernetFrameHeader() = default;
 
     MACAddress destination() const { return m_destination; }
     void set_destination(const MACAddress& address) { m_destination = address; }

Kernel/Net/ICMP.h

@@ -38,8 +38,8 @@ struct ICMPType {
 
 class [[gnu::packed]] ICMPHeader {
 public:
-    ICMPHeader() { }
-    ~ICMPHeader() { }
+    ICMPHeader() = default;
+    ~ICMPHeader() = default;
 
     u8 type() const { return m_type; }
     void set_type(u8 b) { m_type = b; }

Kernel/Net/Socket.h

@@ -180,7 +180,7 @@ private:
 template<typename SocketType>
 class SocketHandle {
 public:
-    SocketHandle() { }
+    SocketHandle() = default;
 
     SocketHandle(NonnullRefPtr<SocketType>&& socket)
         : m_socket(move(socket))

Kernel/Net/UDP.h

@@ -32,8 +32,8 @@ namespace Kernel {
 
 class [[gnu::packed]] UDPPacket {
 public:
-    UDPPacket() { }
-    ~UDPPacket() { }
+    UDPPacket() = default;
+    ~UDPPacket() = default;
 
     u16 source_port() const { return m_source_port; }
     void set_source_port(u16 port) { m_source_port = port; }