xlib_libX11: compile makekeys for the host correctly

As reported by Miroslav Ignjatovic in bug #2983, our hack in
xlib_libX11 to build makekeys for the host does not work, for several
reasons:

 * As we are building a tool for the host, we shouldn't pass
   -I$(STAGING_DIR)/usr/include, since the $(STAGING_DIR) contains
   headers of packages for the target.

 * Instead, we should use the headers in $(HOST_DIR)/usr/include. They
   were not used due to a typo: $(HOST_CFLAGS) must be used instead of
   $(HOSTCFLAGS).

 * Finally, in order for makekeys to find the required headers in
   $(HOST_DIR)/usr/include, we must depend on host-xproto_xproto.

This fixes bug #2983.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>