git: remove a.out and include it in .gitignore

a.out is a configure script byproduct. It was mistakenly checked into
the tree in !13118. This patch removes it, and include it in
.gitignore to prevent a similar error in the future.

(cherry picked from commit f0408eeb)

.gitignore

@@ -35,6 +35,7 @@ Thumbs.db
 __pycache__
 .mypy_cache
 *.SYMDEF*
+a.out
 
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