deps: 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: nodejs#47638

deps/base64/base64/.gitignore

@@ -1,6 +1,5 @@
 *.o
 bin/base64
-lib/config.h
 test/benchmark
 test/test_base64