RISC-V: ignore xipImage

This built file shows up in "git status" without an explicit ignore.

Reviewed-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: palmer-dabbelt

arch/riscv/boot/.gitignore

@@ -4,3 +4,4 @@ Image.*
 loader
 loader.lds
 loader.bin
+xipImage