configs/qemu s390x: increase image size to 120M

The default kernel configuration for s390x enable a lot of
drivers by default so increase the image site to 120M.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4364600444

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 757251a0deeb28cf4a682bb9db5a438be4c0f925)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/qemu_s390x_defconfig

@@ -9,6 +9,7 @@ BR2_SYSTEM_DHCP="eth0"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="board/qemu/post-image.sh"
 BR2_ROOTFS_POST_SCRIPT_ARGS="$(BR2_DEFCONFIG)"
 BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_SIZE="120M"
 # BR2_TARGET_ROOTFS_TAR is not set
 
 # Linux headers same as kernel