Adds `local.exp` to CLEANFILES
Adds `local.exp` to CLEANFILES

With #315, #316, #317, #318 and this patch, running
```
AM_DISTCHECK_CONFIGURE_FLAGS=--disable-docs make distcheck
```
should complete successfully.
plicease/libffi,plicease/libffi,plicease/libffi,plicease/libffi