qt5base: stop qt stripping installed libraries

By default the 'make install' step for qt5base will strip all the binaries
and libraries when they are installed to staging. This hinders debugging
as the libraries in staging dir have been stripped and is unnecessary
as buildroot will strip all files on the target

Signed-off-by: Will Wagner <will_wagner@carallon.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Fatih Aşıcı <fatih.asici@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>