cmake: add install interface include directory

[madebr]

Hello,

The imported target openjp2, created when doing find_package(OpenJPEG REQUIRED CONFIG) does not add the directory include/openjpeg-2.5 to the include path.
This makes doing #include <openjpeg.h> impossible.

After the patch of this pr, the automatically generated OpenJPEGTargets.cmake will contain:

# Create imported target openjp2
add_library(openjp2 SHARED IMPORTED)

set_target_properties(openjp2 PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/openjpeg-2.5"
)

# Create imported target openjp2_static
add_library(openjp2_static STATIC IMPORTED)

set_target_properties(openjp2_static PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/openjpeg-2.5"
)

where before, the include directories were not added.