alpha/boot: Add .gitignore ignoring vmlinux kernel image

Building the kernel creates the untracked kernel image file 'vmlinux' in 'arch/alpha/boot/' - ignore it by adding a new local .gitignore file.

---
 arch/alpha/boot/.gitignore | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 arch/alpha/boot/.gitignore

new file mode 100644
index 000000000000..4abc9c8ab7d3
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+vmlinux

2.51.1

[ Cc the kbuild subsystem mailing list and its maintainers. ]

Hi Thorsten Blum,

(If the maintainers happen to spot patches like this), Please feel free to add my "Co-developed-by" or "Reviewed-by" tag: WangYuli <[email protected]>

 — I would be very grateful!