examples/performance-thread: use a single build dir

When building any of the perf-thread examples, the output .o files were
placed in two separate directories for each app: the regular build dir and
a "common" build directory. This was due to the way the files to be built
were specified, using a relative path. Switching to use VPATH to find the
files causes Make to put all .o's into the one build directory.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>