qextserialport: really disable target (and not staging) install for static builds

qextserialport: really disable target (and not staging) install for static builds

Commit f09b33a0a (qextserialport: fix static build) adjusted the logic for
static builds, but the change contained a typo - It disabled
_INSTALL_STAGING for static builds, not _INSTALL_TARGET.

The autobuilders didn't detect this as nothing links against qextserialport
(so the missing staging install didn't cause issues) and the target install
command was only defined for !static.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>