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

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 <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Fixes: ffef80ecf89f ("perf Documentation: Support for asciidoctor")
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>