stats: Fix the compiler warnings

Do not remove the const, include strings.h for strcmp
diff --git a/src/statistics.c b/src/statistics.c
index d1ffe52..a612140 100644
--- a/src/statistics.c
+++ b/src/statistics.c
@@ -20,8 +20,8 @@
  *
  */
 
-
 #include <sys/types.h>
+#include <string.h>
 
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/core/talloc.h>