tools: don't gitignore base64 config.h

The file is checked into git. Ignoring it causes a very non-obvious way
of breaking tarball builds:

1. Download and unpack tarball
2. Check the sources into git with `git init; git add .; git commit -a`
3. Clean the source tree with `git clean -dfx`
4. Run `./configure && make`
5. Observe build failure because config.h is missing

Fixes: #47638
PR-URL: #48174
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

tools/dep_updaters/update-base64.sh

@@ -62,7 +62,11 @@ mv "$WORKSPACE/base64" "$DEPS_DIR/base64/"
 
 # Build configuration is handled by `deps/base64/base64.gyp`, but since `config.h` has to be present for the build
 # to work, we create it and leave it empty.
-echo "// Intentionally empty" >> "$DEPS_DIR/base64/base64/lib/config.h"
+echo "// Intentionally empty" > "$DEPS_DIR/base64/base64/lib/config.h"
+
+# Clear out .gitignore, otherwise config.h is ignored. That's dangerous when
+# people check in our tarballs into source control and run `git clean`.
+echo "# Intentionally empty" > "$DEPS_DIR/base64/base64/.gitignore"
 
 # update the base64_version.h
 cat > "$BASE_DIR/src/base64_version.h" << EOL