build: fix HAS_GI_DOCGEN never defined error

build: fix HAS_GI_DOCGEN never defined error

When building without --enable-bindings-glib configure reports this error:

  ...
  checking for help2man... true
  checking that generated files are newer than configure... done
  configure: error: conditional "HAS_GI_DOCGEN" was never defined.
  Usually this means the macro was only invoked conditionally.
  make: *** [Makefile:440: config.status] Error 1

Move the initialization of HAS_GI_DOCGEN outside the conditional
with_bindings_glib section so it is always initialized.

Fixes: e090088c21b7 ("bindings: add GLib bindings")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240831075212.147812-1-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>