lib/reed_solomon: Fix array subscript 0 is outside array bounds

lib/reed_solomon: Fix array subscript 0 is outside array bounds

The grub_absolute_pointer() is a compound expression that can only work
within a function. We are out of luck here when the pointer variables
require global definition due to ATTRIBUTE_TEXT that have to use fully
initialized global definition because of the way linkers work.

  static gf_single_t * const gf_powx ATTRIBUTE_TEXT = (void *) 0x100000;

For the reason given above, use GCC diagnostic pragmas to suppress the
array-bounds warning.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>