otbSiftFastImageFilter.h can not be used from outside otb

Description

otbSiftFastImageFilter.hxx has #include "siftfast.h", but this header is installed in ${CMAKE_INSTALL_PREFIX}/otbsiftfast/siftfast.h, which is not in CMake search paths for UseOTB.cmake.

Note that there is a otb_siftfast.h which is installed in the main include dir, but can not be used when using otbSiftFastImageFilter.hxx from within OTB.

Steps to reproduce

#include "otbSiftFastImageFilter.h" in a minimal project that does find_package(OTB REQUIRED).