Fix some warnings

The warnings were:
implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
incompatible implicit declaration of built-in function 'malloc'
implicit declaration of function 'free' [-Wimplicit-function-declaration]
incompatible implicit declaration of built-in function 'free'
diff --git a/netdissect.c b/netdissect.c
index 08a3280..60c0a75 100644
--- a/netdissect.c
+++ b/netdissect.c
@@ -30,6 +30,7 @@
 #include "netdissect.h"
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #ifdef USE_LIBSMI
 #include <smi.h>