chore: Add net_(new|kill)_strerror to cppcheck's allocators.

This helps ensure strerrors are cleaned up everywhere.

other/docker/cppcheck/toxcore.cfg

@@ -92,6 +92,10 @@
     <alloc init="true">new_networking_no_udp</alloc>
     <dealloc arg="1">kill_networking</dealloc>
   </resource>
+  <resource>
+    <alloc init="true">net_new_strerror</alloc>
+    <dealloc arg="1">net_kill_strerror</dealloc>
+  </resource>
   <resource>
     <alloc init="true">new_onion</alloc>
     <dealloc arg="1">kill_onion</dealloc>