The Jenkins "Gerrit Linux clang/dbgutil" builds started to fail in odd ways,
presumably because of having the Developer Toolset in the PATH now when building
compilerplugins.  So change compilerplugins/clang/pluginhandler.hxx slightly to
force rebuild.

Change-Id: I97b8f89dc2e20c85f130284ca5326a5c96c2a1ba
Reviewed-on: https://gerrit.libreoffice.org/63962Tested-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>