glsl/linker: specify proper direction in location aliasing error

glsl/linker: specify proper direction in location aliasing error

The check for location aliasing was always asuming output variables
but this validation is also called for input variables.

Fixes: e2abb75b0e4 ("glsl/linker: validate explicit locations for SSO programs")
Cc: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit 428164d87f8dc1d378236b4913538803653770c6)