utils.h: we need to include utils.h for osmo_generate_backtrace()

this causes compiler warnings in user code ever since commit
55cf02221f0654a3f48888f3b13766b671a120f0
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index c8efce2..2333de6 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -1,6 +1,8 @@
 #ifndef OSMOCORE_UTIL_H
 #define OSMOCORE_UTIL_H
 
+#include <osmocom/core/backtrace.h>
+
 /*! \defgroup utils General-purpose utility functions
  *  @{
  */