package/qt5tools: hide qdoc with llvm dependencies

Building qdoc requires a llvm and clang for the host.

However, there is a limitation in the llvm and clang packages in
Buildroot, which makes it impossible to have a host variant without
a target variant.

So, propagate the dependencies of the target llvm and clang, to ensure
we can only have a host-llvm and -clang packages that are correctly
built.

Note that we do propagate all of the dependencies (instead of just the
architecture part), to be consistent.

Reported-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>