Revert "tests: kernel: xip: exclude qemu_riscv32_xip"

This reverts commit 34159054, the issue
has been fixed in e1647e35, tested with

west build -t run -p -b qemu_riscv32_xip \
	-T tests/kernel/xip/arch.common.xip

west build -t run -p -b qemu_riscv32_xip \
	-T tests/kernel/xip/arch.common.xip.minimallibc

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>