gitignore: don't ignore all aclocal.m4 files

While GHC's own aclocal.m4 is generated by the aclocal tool, other packages' aclocal.m4 are committed in the repository. Previously .gitignore included an entry which covered any file named aclocal.m4, which lead to quite some confusion (e.g. see #21740 (closed)). Fix this by modifying GHC's .gitignore to only cover GHC's own aclocal.m4.