validate: silence g-ir-scanner warnings about GST_IS_VALIDATE_*

Which it complains about because we say our prefix is 'gst_validate' so it should really be GST_VALIDATE_IS_* instead.

Hide the boilerplate defines from g-ir-scanner, it doesn't need to process them.

Fixes #46 (closed)

Questions / Todo

• make sure it's ok for g-ir-scanner to skip these boilerplate defines
• alternative solution would be to declare namespace to be Gst which is what we do in gst-plugins-base