Update PKGBUILD

Update libgcj12 revision

I bumped the package revision to 4.6.3_4 and changed MD5sums, as the libgcj12_4.6.3_3 is not on the Debian mirrors anymore (it returns a 404 error).
Merge pull request #3 from pogliamarci/patch-1

libgcj12: new version