glsl: Add production for "subroutine qualifier"

This covers two of the three uses of 'subroutine':

   - Subroutine type declarations
   - Subroutine uniform declarations

Support for the `subroutine (type, ...)` form will be added in
a later commit.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glsl: Add production for subroutine qualifier with type list

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glsl: Exclude 'subroutine' from has_qualifier()

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glsl: Disallow 'subroutine' as a general type qualifier

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>