backtrace: use namespace prefix osmo_*

Summary of changes:

s/backtrace/osmo_backtrace/g
diff --git a/include/osmocom/core/backtrace.h b/include/osmocom/core/backtrace.h
index bbbb2c2..5a8a816 100644
--- a/include/osmocom/core/backtrace.h
+++ b/include/osmocom/core/backtrace.h
@@ -1,6 +1,6 @@
 #ifndef _OSMO_BACKTRACE_H_
 #define _OSMO_BACKTRACE_H_
 
-void generate_backtrace();
+void osmo_generate_backtrace();
 
 #endif