DEBUG: cpufreq: fix cpu_capacity tracing build for non-smp systems

cpu curr capacity can only be traced for SMP systems. Non-SMP builds
will fail with:

drivers/cpufreq/cpufreq.c: In function ‘cpufreq_freq_transition_begin’:
drivers/cpufreq/cpufreq.c:438:22: error: implicit declaration of function ‘capacity_curr_of’ [-Werror=implicit-function-declaration]
   trace_cpu_capacity(capacity_curr_of(cpu), cpu);
                      ^

Fixes: Change-Id: Icd0930d11068fcb7d2b6a9a48e7ed974904e1081
       ("DEBUG: sched,cpufreq: add cpu_capacity change tracepoint")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>