x86: avoid build warning

fix this build warning:

 include/asm/topology_32.h: In function 'node_to_first_cpu':
 include/asm/topology_32.h:66: warning: unused variable 'mask'

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>