perf Documentation: Fix out-of-tree asciidoctor man page generation

The dependency for the man page rule using asciidoctor incorrectly
specifies a source file in $(OUTPUT).  When building out-of-tree, the
source file is not found, resulting in a fall-back to the following rule
which uses xmlto.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180916151704.GF4765@decadent.org.uk
Fixes: ffef80ec

 ("perf Documentation: Support for asciidoctor")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>