package/gst1-validate: disable introspection

- disable introspection unconditionally (as already done for all
  other original gstreamer1 packages)
- use '=' instead of '+=' for the first usage of GST1_VALIDATE_CONF_OPTS

Fixes:

  http://autobuild.buildroot.net/results/e6e43fb85c71af9bb599ea8bbe2e805b392cf1ad



    GEN      GstValidate-1.0.gir
  Couldn't find include 'GstPbutils-1.0.gir' (search path: '['/nvmedata/autobuild/instance-6/output-1/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/bin/../share/gir-1.0', '/usr/share/gir-1.0', '/usr/share/gir-1.0', '/usr/share/gir-1.0', '/usr/share/gir-1.0', '/usr/share/gir-1.0', '/nvmedata/autobuild/instance-6/output-1/host/share', 'gir-1.0', '/nvmedata/autobuild/instance-6/output-1/host/share/gir-1.0', '/usr/share/gir-1.0']')
  make[5]: *** [Makefile:1612: GstValidate-1.0.gir] Error 1

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>