orcc: use #pragma once in generated headers

Simplifies code and removes build path fragments from
generated file (paths may be different if generated
in a gst-build setup).

There shouldn't be any portability issues with this. Meson
has been using this in its generated config.h for years.

Part-of: <gstreamer/orc!45>
4 jobs for pragma-once-in-generated-headers in 4 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3446011
debian sid

00:01:08

passed #3446014
gstreamer-windows 1809 windows docker
msys2

00:04:11

passed #3446012
gstreamer-windows 1809 windows docker
vs2017 amd64

00:01:05

passed #3446013
gstreamer-windows 1809 windows docker
vs2017 x86

00:01:05