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

gallium/aux/util/u_cpu_detect.h: Fix -Wsign-compare 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)

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