examples: Add missing macro check in perf_counter.h

Check the macro `PKG_USING_PERF_COUNTER`, as done in
`examples/common/platform/lib/perf_os_patch_rt_thread.c`. This avoids
linking failure of every file that includes `perf_counter.h` due to
undefined `__ensure_systick_wrapper`, whose definition is only provided
for RT-Thread (in the aforementioned file) and not other RTOSes or bare
metal.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>