add string.h include to fix compiler warning
diff --git a/src/e1_input_vty.c b/src/e1_input_vty.c
index a8ccabb..aa70805 100644
--- a/src/e1_input_vty.c
+++ b/src/e1_input_vty.c
@@ -20,6 +20,7 @@
 
 #include <stdlib.h>
 #include <unistd.h>
+#include <string.h>
 
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/core/talloc.h>