intel/compiler: Prevent warnings in the following patch

The next patch replaces an unsigned bitfield with a plain unsigned,
which triggers gcc to begin warning on signed/unsigned comparisons.

Keeping this patch separate from the actual move allows bisectablity and
generates no additional warnings temporarily.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.h

@@ -119,7 +119,7 @@ public:
    void setup_payload_interference(struct ra_graph *g, int payload_reg_count,
                                    int first_payload_node);
    int choose_spill_reg(struct ra_graph *g);
-   void spill_reg(int spill_reg);
+   void spill_reg(unsigned spill_reg);
    void split_virtual_grfs();
    bool compact_virtual_grfs();
    void assign_constant_locations();