gallium/aux/util/u_cpu_detect.h: Fix warning in u_cpu_detect.c

Change the type of util_cpu_caps::nr_cpus to int because sysconfig
returns a signed value, fixes:

u_cpu_detect.c: In function 'util_cpu_detect':
u_cpu_detect.c:317:30: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
    if (util_cpu_caps.nr_cpus == -1)

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/gallium/auxiliary/util/u_cpu_detect.h

@@ -46,7 +46,7 @@ extern "C" {
 
 
 struct util_cpu_caps {
-   unsigned nr_cpus;
+   int nr_cpus;
 
    /* Feature flags */
    int x86_cpu_type;