Fix path seperator problem on Windows of Gradle generateCMake task

[cqjjjzr]

This fix changes the path seperator in the CMakeLists.txt generated by generateCMake task of Gradle plugin from '' to '/'.

Without this fix:

CMake Error at CMakeLists.txt:12 (konanc_executable):
  Syntax error in cmake code at

    C:/Users/cqjjj/source/repos/kotlin-libui/CMakeLists.txt:16

  when parsing string

    C:\Users\cqjjj\source\repos\kotlin-libui\build\konan\res\samples.res

  Invalid character escape '\U'.