Fix UK/GB keyboard layout

The UK/GB keyboard doesn't work properly connecting to xrdp with xorgxrdp.
It does work when connecting to xrdp with x11vnc, however.

This is because the layout is not declared in xrdp_keyboard.ini.  It needs
to be called "gb" not "uk".  (There are other layouts that have
km-nnnn.ini files that aren't declared in xrdp_keyboard.ini, so they might
have the same bug, but I haven't tested that.  This is analagous to the
commits for the "ch" and "pl" layouts.)

Test case:

1. Use a PC with the UK/GB keyboard layout.

2. Create /etc/xrdp/startwm.sh as follows:

     #!/bin/sh
     export LANG=en_GB.UTF-8
     export MDM_LANG=en_GB.UTF-8
     export XTERM_LOCALE=en_GB.UTF-8
     xterm
     exit 0

3. Connect using rdesktop to localhost (from a session where the same
   environment variables are defined).

4. Test layout-specific keys such as:
      \ (backslash)
      | (bar)
      # (numbersign)
      ~ (asciitilde)
      £ (sterling: Shift-3)
      € (EuroSign: AltGr-4)

These keys should produce the appropriate symbol but they don't.  With
the change in this commit the keys produce the correct symbol.

xrdp/xrdp_keyboard.ini

@@ -69,6 +69,7 @@ rdp_layout_kr=0x00000412
 rdp_layout_ru=0x00000419
 rdp_layout_se=0x0000041D
 rdp_layout_ch=0x00000807
+rdp_layout_gb=0x00000809
 rdp_layout_pt=0x00000816
 rdp_layout_br=0x00000416
 rdp_layout_pl=0x00000415
@@ -84,6 +85,7 @@ rdp_layout_kr=kr
 rdp_layout_ru=ru
 rdp_layout_se=se
 rdp_layout_ch=ch
+rdp_layout_gb=gb
 rdp_layout_pt=pt
 rdp_layout_br=br(abnt2)
 rdp_layout_pl=pl
@@ -113,6 +115,7 @@ rdp_layout_it=it
 rdp_layout_ru=ru
 rdp_layout_se=se
 rdp_layout_ch=ch
+rdp_layout_gb=gb
 rdp_layout_pt=pt
 rdp_layout_br=br(abnt2)
 rdp_layout_pl=pl