package/qt5/qt5base: fix libatomic handling to not affect host builds

qt5declarative builds qmltyperegistrar for the host as part of its build
process.
When building qt target packages (which is the case for qt5declarative),
-spec devices/linux-buildroot-g++ is passed to qmake in QT5_QMAKE
variable and this spec currently has -latomic in its LIBS.

This -latomic makes it to the build of the host build of
qmltyperegistrar which is not useful.

This was discovered on Fedora 34 where libatomic is not pulled with gcc
package, therefore was missing on the host machine.

This makes sure that -latomic is not added for host build of qt
packages.

Fixes: 7d286be4 ("package/qt5base: link with -latomic when needed")

Cc: Quentin Schulz <foss@0leil.net>
Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>