glsl: Fix ir_print_visitor's handling of interpolation qualifiers.

This patch updates the interp[] array to match the enum
glsl_interp_qualifier.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

v2: Add a STATIC_ASSERT to make sure the array is the correct size.
This required adding INTERP_QUALIFIER_COUNT to the enum.