perf: Fix compilation without libelf for post-3.10 kernels

kernel commit cf4cca10 (perf tools: Move libelf check config into config/Makefile)
removes the NO_LIBELF check from the top-level Makefile

for newer kernels, if config/Makefile exists, we can assume that NO_LIBELF is
also there

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>