closes https://github.com/assimp/assimp/issues/1426: add Defines.h to include folder for install.

code/CMakeLists.txt

@@ -61,6 +61,7 @@ SET( PUBLIC_HEADERS
   ${HEADER_PATH}/color4.inl
   ${CMAKE_CURRENT_BINARY_DIR}/../include/assimp/config.h
   ${HEADER_PATH}/defs.h
+  ${HEADER_PATH}/Defines.h
   ${HEADER_PATH}/cfileio.h
   ${HEADER_PATH}/light.h
   ${HEADER_PATH}/material.h