Use "#pragma once" instead of preprocessor guards

Guards currently are quite different in format.
Unify updating to "#pragma once" syntax. This syntax is used
by GTK tools too.
To avoid possible regression public headers are left untouched.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Snir Sheriber <ssheribe@redhat.com>
2 jobs for master in 3 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #495033
fedora

00:03:13

passed #495034
windows

00:01:52