fix: add missing chdir entry in FAT32 jump tables

The dir_chdir entry was missing from both the fat namespace virtual
table (f256/fat32.asm) and the f256jr jump table (fat32/f256jr.s),
causing a build failure.

Author: mbrukner

f256/fat32.asm

@@ -52,6 +52,8 @@ dir_close   .fill   3
 
 dir_mkdir   .fill   3
 dir_rmdir   .fill   3
+
+dir_chdir   .fill   3
             .endn
             .endv                   
 

fat32/f256jr.s

@@ -60,6 +60,8 @@ used:   .res    1
         jmp     fat32_mkdir
         jmp     fat32_rmdir
 
+        jmp     fat32_chdir
+
 get_error:
         lda     fat32_errno
         rts