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_vec4.h

@@ -132,7 +132,7 @@ public:
    bool reg_allocate();
    void evaluate_spill_costs(float *spill_costs, bool *no_spill);
    int choose_spill_reg(struct ra_graph *g);
-   void spill_reg(int spill_reg);
+   void spill_reg(unsigned spill_reg);
    void move_grf_array_access_to_scratch();
    void move_uniform_array_access_to_pull_constants();
    void move_push_constants_to_pull_constants();