Impact: clean, reduce kernel size a bit, avoid sparse warnings

Fixes sparse warnings:

 arch/x86/kernel/cpu/perf_counter.c:153:6: warning: symbol 'hw_perf_enable_all' was not declared. Should it be static?
 arch/x86/kernel/cpu/perf_counter.c:279:3: warning: returning void-valued expression
 arch/x86/kernel/cpu/perf_counter.c:206:3: warning: returning void-valued expression
 arch/x86/kernel/cpu/perf_counter.c:206:3: warning: returning void-valued expression
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>