Add backend.h for backend interfaces
diff --git a/libltrace.c b/libltrace.c
index a49db28..f4bb2c8 100644
--- a/libltrace.c
+++ b/libltrace.c
@@ -33,6 +33,7 @@
 #include "common.h"
 #include "proc.h"
 #include "read_config_file.h"
+#include "backend.h"
 
 char *command = NULL;