coin: Skip QtPdf on the HarmonyOS x86_64 target

The harmonyos-26-x86_64-on-linux configuration builds QtPdf for OHOS
on x86_64, which Chromium's GN setup doesn't handle yet: GN generation
stops at an "Unhandled cpu type" assertion in
build/config/clang/BUILD.gn. Because the configuration applies to every
module that pulls in qtwebengine, integrations in qtwebengine and its
dependents, such as qtdoc, fail regardless of the change under test.

Disable the QtWebEngine and QtPdf builds for this configuration's
non-qtbase target builds, as the Windows ARM64EC configuration already
does, until QtWebEngine supports OHOS on x86_64.

Task-number: QTBUG-150347
Fixes: QTBUG-151373
Change-Id: I42d37086122eead72642db84721b9488a84db1c8
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>