Migrate from #ifndef guards to #pragma once

Using a pragma instead of guards is easier to write, less error prone
and avoids name clashes (a source of very subtle bugs). This pragma
is supported on almost all compilers, including all the compilers we
care about: https://en.wikipedia.org/wiki/Pragma_once#Portability.

This patch does not change the autogenerated files: asn1/asn1c/*.h.

Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>

util/ipa_krb5.h

@@ -1,5 +1,4 @@
-#ifndef __IPA_KRB5_H_
-#define __IPA_KRB5_H_
+#pragma once
 
 #include <lber.h>
 #include <krb5/krb5.h>
@@ -83,4 +82,3 @@ int create_keys(krb5_context krbctx,
                 char **err_msg);
 
 int ipa_kstuples_to_string(krb5_key_salt_tuple *kst, int n_kst, char **str);
-#endif /* __IPA_KRB5_H_ */