mesa/gallium: do not use enum for bit-allocated member

The signedness of enums are undefined, so on platforms with signed
enums, this isn't going to work. One such platform is Microsoft Windows.

So let's just use an unsigned here instead.

Fixes: b1c4c4c7f5 ("mesa/gallium: automatically lower alpha-testing")
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4648>

src/mesa/state_tracker/st_program.h

@@ -137,7 +137,7 @@ struct st_fp_variant_key
    GLuint lower_two_sided_color:1;
 
    GLuint lower_flatshade:1;
-   enum compare_func lower_alpha_func:3;
+   unsigned lower_alpha_func:3;
 
    /** needed for ATI_fragment_shader */
    char texture_targets[MAX_NUM_FRAGMENT_REGISTERS_ATI];