cpumask: fix function description kernel-doc notation

Use kernel-doc notation for the function description to prevent
a warning:

lib/cpumask.c:160: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Returns an arbitrary cpu within srcp1 & srcp2.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>