When we do an in-tree-build with autotools and an out-of-tree build
with meson (all in the same source directory), then we have the
following files:

  libnm-core/nm-core-enum-types.h
  libnm/nm-enum-types.h
  build/libnm-core/nm-core-enum-types.h
  build/libnm/nm-enum-types.h

This caused "tools/check-docs.sh" for `ninja -C build test` to fail,
because the files are detected twice:

    --- command ---
    /data/src/NetworkManager/tools/check-docs.sh /data/src/NetworkManager /data/src/NetworkManager/build
    --- stderr ---
    8a9
    > nm-core-enum-types
    38a40
    > nm-enum-types
    *** Error: libnm classes not included in docs/libnm/libnm-docs.xml ***
    -------

(cherry picked from commit 17adf58d)