package/qt5: enable for nios2

The double-conversion support for nios2 is already enabled
by commit [1], so drop dependency on !BR2_nios2.

[1] https://git.buildroot.net/buildroot/commit/?id=e2fdb41f711db8894f9c5c83f32250728d4c3aa9



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>