Do not include farsi.c in main.c

- Change CMakeLists.txt to compile farsi.c normally
 - Add const to global variables in farsi.h and define them in farsi.c (no need
   to include farsi.h with DO_INIT defined in globals.h)
 - Include farsi.h where necessary
 - Include all necessary headers in farsi.c
 - Move farsi function declarations from main.h to farsi.h

src/charset.c

@@ -4,6 +4,7 @@
 
 #include "vim.h"
 #include "charset.h"
+#include "farsi.h"
 #include "main.h"
 #include "mbyte.h"
 #include "memline.h"