add exec_wm to calmwm.h, it was missing (bad ian@!)

Also, exec_wm() currenty err()s out if something failed with the exec,
killing the currently running wm. This is bad. Replace the err() with
warn() instead.


from Gleydson Soares, thanks!

calmwm.h

@@ -396,6 +396,7 @@ int dirent_exists(char *);
 int dirent_isdir(char *);
 int dirent_islink(char *);
 int u_spawn(char *);
+void exec_wm(char *);
 
 void  grab_sweep(struct client_ctx *);
 void  grab_drag(struct client_ctx *);