adjust include path in pkg-config for OpenANN

Add -I${prefix}/include  (this results in #include <OpenANN/...>)
Add -I${prefix}/include/OpenANN (for the OpenANN library itself,
that references to the headers without the OpenANN directory)

Signed-off-by: Chris Mueller <christoph.mueller@dfki.de>
Merge pull request #2 from Ruun/bugfixes

adjust include path in pkg-config for OpenANN