x86: cpu: Add missing header inclusion

x86: cpu: Add missing header inclusion

Without asm/cpu_x86.h inclusion a compiler is not happy:

  arch/x86/cpu/cpu_x86.c:14:5: warning: no previous prototype for ‘cpu_x86_bind’ [-Wmissing-prototypes]
  arch/x86/cpu/cpu_x86.c:29:5: warning: no previous prototype for ‘cpu_x86_get_vendor’ [-Wmissing-prototypes]
  arch/x86/cpu/cpu_x86.c:41:5: warning: no previous prototype for ‘cpu_x86_get_desc’ [-Wmissing-prototypes]
  arch/x86/cpu/cpu_x86.c:55:5: warning: no previous prototype for ‘cpu_x86_get_count’ [-Wmissing-prototypes]

Add missing header inclusion.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>