qt: make $(HOST_DIR)/usr/mkspecs a symlink to $(STAGING_DIR)/usr/mkspecs

In fact, not only qt can install stuff in $(STAGING_DIR)/usr/mkspecs,
but also libraries that wish to integrate with qmake, such as
qwt. However, qmake and al. will look inside $(HOST_DIR)/usr/mkspecs,
so the easiest solution is to have a symbolic link between the two
directories, instead of copying things around.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>