gdb-ui.el assumes images support

[Eli Zaretskii]

This line from gdb-ui.el:

  (define-fringe-bitmap 'breakpoint
         "\x3c\x7e\xff\xff\xff\xff\x7e\x3c")

causes the Emacs build to fail if it is configured --without-x, since
fringe.c is not compiled then.  (gdb-ui.el affects the build because
tooltip.el says "(require 'gdb-ui)".)

I fixed it temporarily by conditioning define-fringe-bitmap by images
being supported, but that's a kludge, as it breaks the possibility to
prepare an Emacs tarball on a system where Emacs is built without X,
which is typical of making a tarball on fencepost.  So a better change
is required (why does tooltip.el need to require gdb-ui, anyway?).