include: Switch to #pragma once pattern

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/include/osmocom/core/linuxrbtree.h b/include/osmocom/core/linuxrbtree.h
index 079f440..ef8bc15 100644
--- a/include/osmocom/core/linuxrbtree.h
+++ b/include/osmocom/core/linuxrbtree.h
@@ -91,8 +91,7 @@
 -----------------------------------------------------------------------
 */
 
-#ifndef	_LINUX_RBTREE_H
-#define	_LINUX_RBTREE_H
+#pragma once
 
 #include <stdlib.h>
 
@@ -156,5 +155,3 @@
 
 	*rb_link = node;
 }
-
-#endif	/* _LINUX_RBTREE_H */