perf_counter tools: Fix off-by-one bug in symbol__new

The end is really (start + len - 1). Noticed when synthesizing
the PLT symbols, that are small (16 bytes), and hot on the
start RIP.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20090603174921.GG7805@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>