math/fann: fix plist without libomp

pkg-static: Unable to access file /wrkdirs/usr/ports/math/fann/work/stage/usr/local/include/parallel_fann.h:No such file or directory
pkg-static: Unable to access file /wrkdirs/usr/ports/math/fann/work/stage/usr/local/include/parallel_fann.hpp:No such file or directory