fix autoreconf error with homebrew

```
+ cd libssh2
+ rm -r -f x64-musl bld
+ '[' -f configure ']'
+ autoreconf --force --install
Can't exec "aclocal": No such file or directory at /usr/local/Cellar/autoconf/2.71/share/autoconf/Autom4te/FileUtils.pm line 274.
autoreconf: error: aclocal failed with exit status: 2
```
Ref: https://github.com/vszakats/curl-for-win/actions/runs/6392970919/job/17351293395#step:3:4308

_ci-mac-homebrew.sh

@@ -6,7 +6,7 @@
 # shellcheck disable=SC3040,SC2039
 set -o xtrace -o errexit -o nounset; [ -n "${BASH:-}${ZSH_NAME:-}" ] && set -o pipefail
 
-extra=''
+extra='automake'
 [[ "${CW_CONFIG:-}" = *'boringssl'* ]] && extra="${extra} go"
 [[ "${CW_CONFIG:-}" != *'mac'* ]] && extra="${extra} llvm"