perf build: Fix out of tree build related to installation of sysreg-defs

perf build: Fix out of tree build related to installation of sysreg-defs

It seems that a previous modification to sysreg-defs, which corrected
emitting the header to the specified output directory, exposed missing
subdir, prefix variables.

This breaks out of tree builds of perf as the file is now built into the
output directory, but still tries to descend into output directory as a
subdir.

Fixes: a29ee6aea7030786 ("perf build: Ensure sysreg-defs Makefile respects output dir")
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Ethan Adams <j.ethan.adams@gmail.com>
Tested-by: Tycho Andersen <tycho@tycho.pizza>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240314222012.47193-1-j.ethan.adams@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>