flann: bump version to fix build with gcc 6

The current version of flann in Buildroot fails to build with gcc 6.x,
with the following failure:

  error: call of overloaded 'abs(flann::KDTreeIndex<flann::L2<float> >::ElementType)' is ambiguous

By bumping the upstream version one commit further, we get an upstream
fix that fixes the build problem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>