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