Fix clang unused function/variable errors

With clang and -Werrors, the library did not compile.
Fix both issues:
	- unused perf_get_ovfl_umask_idx() with CONFIG_PFMLIB_NOTRACEPOINT set
	- unused variable sum in self_smpl_multi.c

Signed-off-by: Stephane Eranian <eranian@gmail.com>
2 files changed