fbdev: amifb: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper
Replacing zero-length arrays with C99 flexible-array members
because they are deprecated. Use the new DECLARE_FLEX_ARRAY()
auxiliary macro instead of defining a zero-length array.
This fixes warnings such as:
./drivers/video/fbdev/amifb.c:690:6-10: WARNING use flexible-array member instead