apt-helper.cc: include <stdlib.h> for atoi

Include <stdlib.h> to ensure that atoi(3) is defined to improve
general portability and fix a specific build failure on Android.

Closes: 807031

cmdline/apt-helper.cc

@@ -29,6 +29,8 @@
 #include <string>
 #include <vector>
 
+#include <stdlib.h>
+
 #include <apti18n.h>
 									/*}}}*/