tools: Fix shadowed declaration in err.h

The error variable breaks build on CentOS 6.7, due to collision with
global error symbol:

    CC       util/evlist.o
  cc1: warnings being treated as errors
  In file included from util/evlist.c:28:
  tools/include/linux/err.h: In function ‘ERR_PTR’:
  tools/include/linux/err.h:34: error: declaration of ‘error’ shadows a global declaration
  util/util.h:135: error: shadowed declaration is here

Using 'error_' name instead to fix it.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/n/tip-i9mdgdbrgauy3fe76s9rd125@git.kernel.org
Reported-by: Vinson Lee <vlee@twopensource.com>
[ Use 'error_' instead of 'err' to, visually, not diverge too much from include/linux/err.h ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>