include: Switch to #pragma once pattern

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/include/osmocom/core/backtrace.h b/include/osmocom/core/backtrace.h
index a24290c..0c9b6ef 100644
--- a/include/osmocom/core/backtrace.h
+++ b/include/osmocom/core/backtrace.h
@@ -1,7 +1,4 @@
-#ifndef _OSMO_BACKTRACE_H_
-#define _OSMO_BACKTRACE_H_
+#pragma once
 
 void osmo_generate_backtrace(void);
 void osmo_log_backtrace(int subsys, int level);
-
-#endif