perf tools: Remove string.h from util.h

Not needed in this header, added to the places that need strdup,
strcmp and a few other prototypes.

Link: http://lkml.kernel.org/n/tip-t24yy85xnlv55kyosrum2ubs@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>