[win32] fix library header files installation
FileTimeToTimeT() is only needed on posix platforms
[win32] Int32x32To64 is already available as a macro
cmake: add win32-specific windows sub-directory to the include paths (due to dlfcn-win32.h)
cmake: add proper kodiplatform_INCLUDE_DIRS
[win32] cmake: set proper kodiplatform_LIBRARY
add missing includes to StringUtils.cpp