configs/raspberrypi0: increase ext4 image size

configs/raspberrypi0: increase ext4 image size

Since the bump to Linux 4.19 in commit
ed02414e9cd271c0b0db26b8d5db1bf40ff416d3, the number of kernel modules
built has increased significantly, to the point where the default of
60 MB for the ext4 image is no longer sufficient, so let's increase to
120 MB.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339555

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>