include: Switch to #pragma once pattern

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/include/osmocom/core/timer.h b/include/osmocom/core/timer.h
index d37af80..d734357 100644
--- a/include/osmocom/core/timer.h
+++ b/include/osmocom/core/timer.h
@@ -26,8 +26,7 @@
  *  \brief Osmocom timer handling routines
  */
 
-#ifndef TIMER_H
-#define TIMER_H
+#pragma once
 
 #include <sys/time.h>
 
@@ -85,5 +84,3 @@
 int osmo_timers_check(void);
 
 /*! @} */
-
-#endif