LibKeyboard: Don't compile CharacterMap:set_system_map in kernel mode

In preparation for using Userspace<T> in Syscall::SC_setkeymap_params
remove the usage of SC_setkeymap_params from when compiling in kernel
mode. In kernel model we would need to do a bunch of explicit FlatPtr
cats to in order to get it to compile, and it's unused anyway, so just
avoid the pain.

Libraries/LibKeyboard/CharacterMap.h

@@ -37,7 +37,10 @@ class CharacterMap {
 public:
     CharacterMap(const String& file_name);
 
+#ifndef KERNEL
     int set_system_map();
+#endif
+
     u32 get_char(KeyEvent);
     void set_character_map_data(CharacterMapData character_map_data);