glsl: add missing explicit_image_format flag to has_layout()

Fixes piglit regression after fixes to duplicate layout rules.

Previously catching multiple layouts was relying on the code
meant to catch duplicates within a single layout(...), this
change triggers the rules for multiple layouts.

Cc: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/glsl/ast_type.cpp

@@ -74,6 +74,7 @@ ast_type_qualifier::has_layout() const
           || this->flags.q.row_major
           || this->flags.q.packed
           || this->flags.q.explicit_location
+          || this->flags.q.explicit_image_format
           || this->flags.q.explicit_index
           || this->flags.q.explicit_binding
           || this->flags.q.explicit_offset