Compilation error since change in walkergrid on OS X

[Marc Saurfelt]

Since walker grid has been moved to its own file, I can't compile from source (I did a "make clean" to be sure). The error I have is :

/Users/cram/Applications/caesaria/caesaria/source/city/walkergrid.cpp:25:25: error: default initialization of an object of const type 'const WalkerList' (aka 'const SmartList<Walker>') requires a user-provided default constructor

static const WalkerList invalidList;

[dalerank]

fix #345. fix clang compilation error

→ <<cset adb4af0e42cb>>