experiments-config.h is hiding the xnnpack.h header

The commit
https://github.com/google/XNNPACK/commit/c7e7cde37615a81a529c326aa278bfab4cd6fe5a

made `experiments-config.h` public at the expense of `xnnpack.h`.

I think there there can only be on `public_headers` property for each target.

Looking at the file, I think it would be better to add a subdirectory
```
${CMAKE_INSTALL_INCLUDEDIRS}/xnnpack/experiments-config.h
```
when the user installs this particular header.

The name doesn't really suggest that it is part of the library