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>

client/ipa-client-common.h

@@ -17,8 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef __IPA_CLIENT_COMMON_H
-#define __IPA_CLIENT_COMMON_H
+#pragma once
 
 #include <libintl.h>
 #define _(STRING) gettext(STRING)
@@ -29,5 +28,3 @@
 #endif
 
 int init_gettext(void);
-
-#endif /* __IPA_CLIENT_COMMON_H */