perf tools: Fix build breakage if prefix= is specified

Invoking Makefile.perf with prefix= breaks the build since Makefile.perf
hands that variable down to Makefile.build where it overrides

    prefix       := $(subst ./,,$(OUTPUT)$(dir)/)

leading to errors like this:

    No rule to make target '/usrabspath.o', needed by '/usrlibperf-in.o'

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Fixes: c819e2cf
Link: http://lkml.kernel.org/r/5582c48a.84a22b0a.a918.5285SMTPIN_ADDED_MISSING@mx.google.com


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