CPack/NuGet: Fix nuspec file generation for UNIX-style OS

Use forward slashes, which should work on both UNIX and Windows.

Modules/Internal/CPack/CPackNuGet.cmake

@@ -332,7 +332,7 @@ endfunction()
 function(_cpack_nuget_make_files_tag)
     set(_files)
     foreach(_comp IN LISTS ARGN)
-        string(APPEND _files "        <file src=\"${_comp}\\**\" target=\".\" />\n")
+        string(APPEND _files "        <file src=\"${_comp}/**\" target=\".\" />\n")
     endforeach()
     set(_CPACK_NUGET_FILES_TAG "<files>\n${_files}    </files>" PARENT_SCOPE)
 endfunction()