glsl: Add an is_declaration field to ast_struct_specifier.

glsl: Add an is_declaration field to ast_struct_specifier.

Will be used in a later commit to differentiate between a structure type
declaration and a variable declaration of a struct type. I.e., the
difference between

   struct S { float x; }; (is_declaration = true)

and

   S s;                   (is_declaration = false)

Also note that is_declaration = true for

   struct S { float x; } s;

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>