perf callchain: Move the callchain_param extern to callchain.h

perf callchain: Move the callchain_param extern to callchain.h

It was lost in hist.h, move it to where it belongs, callchain.h, as
there are places that gets hist.h by means of evsel.h, and since evsel.h
is being untangled from hist.h...

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-0rg7ji1jnbm6q6gj35j37jby@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>