CMakeLists.txt: Rename ASSIMP_LIB to ASSIMP_LIBRARIES
CMakeLists.txt: Rename ASSIMP_LIB to ASSIMP_LIBRARIES

The latter name is used by the upstream cmake-config file, and will be
used in the next patch.

grim210/Vulkan,jherico/Vulkan,jherico/Vulkan,ming4883/Vulkan,dreadwords/Vulkan,SaschaWillems/Vulkan,jpbruyere/Vulkan,jaakkowork/Vulkan,SaschaWillems/Vulkan,SaschaWillems/Vulkan,SaschaWillems/Vulkan,SaschaWillems/Vulkan,martty/Vulkan,ming4883/Vulkan,ming4883/Vulkan,jaakkowork/Vulkan,grim210/Vulkan,jpbruyere/Vulkan