weston-wfits: pointer/keyboard are not pointers in older Weston SDK's

For backwards compatibility we can't do a pointer NULL check on
seat->keyboard or seat->pointer since those members were not pointers
in older Weston SDK's.  We only initialize them primarily to make
the extension work on the headless backend (which doesn't init a
pointer/keyboard), thus use isHeadless() as the condition for
initialization.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>