beaglebone_qt5_defconfig: use gcc 5.x

The linux 4.1 kernel does not compile with gcc 6.x.
The compiler version is now fixed to 5.x until a new kernel is used.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>