Add mingw32-libpcre as dependency

This enables us to compile git against libpcre.
And this makes thing like
```
git grep -P "\bpcre\b" HEAD
```
possible.
Which I already did in https://github.com/t-b/git/tree/use-libpcre. And all tests still pass with that too.
Entries should either go to `RequiredPackages` *or* to `RecommendedPackages`, but not both. If I understand you correctly, `mingw32-libpcre` is not *required* to build Git, but it's an (optional) direct dpendency, and as such it should still go to `RequiredPackages`.