configs/raspberrypi3_qt5we: increase filesystem size

configs/raspberrypi3_qt5we: increase filesystem size

The build of raspberrypi3_qt5we_defconfig currently fails because the
filesystem image size is no longer large enough to contain everything:
the filesystem image size is 360 MB, but the total filesystem size
reported by "make graph-size" is 370 MB.

Let's increase the size to 400 MB for good measure.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/389451889

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>