.gitignore: add build-aux/ltmain.sh-e #998

When running autogen in LibWally, which uses secp256k1-zkp as a submodule, this file is left on my system. It also happens when I swap out secp256k1-zkp for secp256k1.

It does not happen when I compile secp256k1 directly, so perhaps this is an issue on the LibWally side.

cc @jgriffiths

I think what's happening here is that some sed variant is misinterpreting the command line arguments.

https://github.com/ElementsProject/libwally-core/blob/master/tools/autogen.sh#L7 reads:

sed -i -e "s/\$AR $a /ar $a /" $f

Now my GNU sed has in the man page:

       -i[SUFFIX], --in-place[=SUFFIX]

              edit files in place (makes backup if SUFFIX supplied)

So I believe the -i -e is somehow wrongly parsed as --in-place='-e', or "edit $f (=ltmain.sh) in place but create a backup file with suffix '-e'". Oo

Sed on macOS is famous for misinterpreting. Think Different, was Apple's motto. Maybe the script should call gsed instead, which is already a requirement IIUC: https://github.com/ElementsProject/libwally-core/blob/master/configure.ac#L95-L104