perf data: Add doc when no conversion support compiled

This adds documentation on the environment variables needed to the
message telling that no conversion support is compiled in.

Committer testing:

  $ make -C tools/perf install
  $ perf data convert --all --to-ctf myctftrace
  No conversion support compiled in. perf should be compiled with environment variables LIBBABELTRACE=1 and LIBBABELTRACE_DIR=/path/to/libbabeltrace/
  $
Signed-off-by: Geneviève Bastien <gbastien@versatic.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Francis Deslauriers <francis.deslauriers@efficios.com>
Cc: Julien Desfossez <jdesfossez@efficios.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170727181205.24843-3-gbastien@versatic.netSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-data.c

@@ -69,7 +69,7 @@ static int cmd_data_convert(int argc, const char **argv)
 	};
 
 #ifndef HAVE_LIBBABELTRACE_SUPPORT
-	pr_err("No conversion support compiled in.\n");
+	pr_err("No conversion support compiled in. perf should be compiled with environment variables LIBBABELTRACE=1 and LIBBABELTRACE_DIR=/path/to/libbabeltrace/\n");
 	return -1;
 #endif