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.

Fixes: ffef80ecf89f ("perf Documentation: Support for asciidoctor")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name perf-documentation-fix-out-of-tree-asciidoctor-man-page-generation.patch