Fix implicit conversion warning in GEBP kernel's packing

Reference issue

Warning 1 in #2400 (closed)

What does this implement/fix?

very small change to remove a shortening conversion warning in GeneralBlockPanelKernel.h. Not too important to fix, but I've seen this warning pop up a number of times in unrelated compilation errors and it's a little annoying to scroll past.

(Since it doesn't really show up in the diff,) gemm_pack_lhs in the gebp kernel includes some code like this

int pack = Pack1;          
int psize = PacketSize; 
... 
Index left = rows - i;      // rows remaining to pack 
...
psize = pack = left & ~1;   // triggers -Wshorten-64-to-32

This MR just changes pack, psize, (and a loop variable) to Index to remove the warning.

Additional Notes

maybe it's preferable to just do int left = internal::convert_index<int>(rows - i)?
I think left = rows - i is the few remaining rows that didn't fit with the current value of pack. So left should definitely be small enough to fit in an int. (and just printing the value of left in the product_large tests confirms it's always smaller than Pack1)