strrb.c: Use string.h, not strings.h

Not all systems have strings.h
diff --git a/src/strrb.c b/src/strrb.c
index 972d130..8f925bd 100644
--- a/src/strrb.c
+++ b/src/strrb.c
@@ -27,7 +27,7 @@
 
 #include <stdio.h>
 #include <string.h>
-#include <strings.h>
+#include <string.h>
 
 #include <osmocom/core/strrb.h>