glsl: consistently use ifndef guards over pragma once

glsl: consistently use ifndef guards over pragma once

Through the glsl headers we had an odd mix of guards be that
"ifndef", "pragma once" neither or both.

Simplify things by using the more common ones (ifndef) and annotating
all the sources, barring the generated builting header -
builtin_int64.h.

The final header - udivmod64.h - is [seemingly] unused and on its way
out (patch purge it is on the mailing list).

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Vedran Miletić <vedran@miletic.net>
Acked-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>