nir: consistently use ifndef guards over pragma once

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>

src/compiler/nir/nir.h

@@ -25,7 +25,8 @@
  *
  */
 
-#pragma once
+#ifndef NIR_H
+#define NIR_H
 
 #include "util/hash_table.h"
 #include "compiler/glsl/list.h"
@@ -2634,3 +2635,5 @@ gl_system_value nir_system_value_from_intrinsic(nir_intrinsic_op intrin);
 #ifdef __cplusplus
 } /* extern "C" */
 #endif
+
+#endif /* NIR_H */