nir: Add explicit_binding to nir_variable

This is copied from the corresponding value in ir_variable. The
intention is to eventually use it in a pure-NIR linker.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/glsl_to_nir.cpp

@@ -435,6 +435,7 @@ nir_visitor::visit(ir_variable *ir)
    var->data.index = ir->data.index;
    var->data.descriptor_set = 0;
    var->data.binding = ir->data.binding;
+   var->data.explicit_binding = ir->data.explicit_binding;
    var->data.bindless = ir->data.bindless;
    var->data.offset = ir->data.offset;
    var->data.image.read_only = ir->data.memory_read_only;