gdb: depend on host-expat to fix remote debugging

This patch fixes the following error when using GDB with gdbserver:

  warning: Can not parse XML target description; XML support was disabled at compile time
  Remote 'g' packet reply is too long: <very long line of hex chars>
  [remote debugging does not work]

Use $(HOST_CONFIGURE_OPTS) so expat is found.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>