Fix posix.mak

Fixes this error:
```
$ make -f posix.mak install
...
$ make -f posix.mak install
...
ln -s libphobos2.so.0.66.0 ../install/linux/lib64/libphobos2.so
ln: failed to create symbolic link '../install/linux/lib64/libphobos2.so': File exists
```