Fix FMT_INSTALL with FMT_MODULE

[yujincheng08]

Error:

  install TARGETS target fmt is exported but not all of its interface file
  sets are installed

[vitaut]

Thanks for the PR. Mostly looks good but I wonder if the include directory is the right place to install sources.

[yujincheng08]

@vitaut basically there's no "standard" place to place the install sources. So it's free to place anywhere, as long as cmake can help export the correct FILE_SET to dependants

[vitaut]

Merged, thanks!