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.
Changed.
We have not yet any dependencies in the git package xml file. I started that work in https://github.com/t-b/sdk-packages/commit/1845e93f90795104498263786a321f4eba549071.