Invalid path separator in QtBuildInternalsExtra.cmake

Backslashes in the path at the following location causes CMake to fail. Replacing them with forward slashes fixes the problem.

    set(qtbi_orig_prefix "C:\Qt")
    set(qtbi_orig_staging_prefix "C:\Qt\6.5.3\wasm_singlethread")

(also in wasm_multithreaded)

The error message in Qt Creator is:

CMake Error at C:/Qt/6.5.3/wasm_singlethread/lib/cmake/Qt6BuildInternals/QtBuildInternalsExtra.cmake:45 (set):
  Syntax error in cmake code at    C:/Qt/6.5.3/wasm_singlethread/lib/cmake/Qt6BuildInternals/QtBuildInternalsExtra.cmake:45  when parsing string    C:\Qt  Invalid character escape '\Q'.{}