perf tools: Fix version file for perf documentation with OUTPUT variable set

perf tools: Fix version file for perf documentation with OUTPUT variable set

Fixes the following:

 + make OUTPUT=/.../.build/perf-user/ DESTDIR=/.../.install/perf-user/ man install-man
 make -C Documentation man
 make[1]: Entering directory `/.../.source/linux.perf/tools/perf/Documentation'
 make[2]: Entering directory `/.../.source/linux.perf/tools/perf'
 make[2]: *** No rule to make target `PERF-VERSION-FILE'.  Stop.

Signed-off-by: Robert Richter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>