Fix CMake problem on windows

When building on Windows I got such message:

C:\Users\Roman\Documents\GitHub\opencaesar3\build>cmake ..
CMake Error at cmake/Modules/FindPackageHandleStandardArgs.cmake:97
(message):
Could NOT find PNG (missing: PNG_PNG_INCLUDE_DIR)
Call Stack (most recent call first):
cmake/Modules/FindPackageHandleStandardArgs.cmake:291
(_FPHSA_FAILURE_MESSAGE)

cmake/Modules/FindPNG.cmake:90 (find_package_handle_standard_args)
CMakeLists.txt:17 (find_package)

-- Configuring incomplete, errors occurred!

This commit fixes it