Supress a compilation warning.

i3wm-delegate.c: In function 'i3wm_goto_workspace':
i3wm-delegate.c:124:36: warning: incompatible implicit declaration of built-in function 'malloc'
     gchar *command_str = (gchar *) malloc(cmd_size);
Merge pull request #5 from xuhdev/supress-malloc-warning

Supress a compilation warning.