gallium/aux/util: Fix some warnings

gallium/aux/util: Fix some warnings

util/u_cpu_detect.c: In function 'util_cpu_detect':
util/u_cpu_detect.c:377:30: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    if (util_cpu_caps.nr_cpus == ~0u)
                              ^~

util/u_hash_table.c:274:21: warning: unused parameter 'k' [-Wunused-
parameter]
 util_hash_inc(void *k, void *v, void *d)
                     ^
util/u_hash_table.c:274:30: warning: unused parameter 'v' [-Wunused-
parameter]
 util_hash_inc(void *k, void *v, void *d)
                              ^

util/u_tests.c: In function 'test_texture_barrier':
util/u_tests.c:652:25: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
       for (int i = 0; i < num_samples / 2; i++) {
                         ^

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>