allow magicmime on windows

John Thorvald Wodder II got it working using
https://github.com/datalad/file-windows so don't hard-disable it.

The stack.yaml still disables this build flag, because it needs an extra
C library to be installed, which stack cannot automate.

git-annex.cabal

@@ -595,9 +595,8 @@ Executable git-annex
     CPP-Options: -DWITH_TORRENTPARSER
 
   if flag(MagicMime)
-    if (! os(windows))
-      Build-Depends: magic
-      CPP-Options: -DWITH_MAGICMIME
+    Build-Depends: magic
+    CPP-Options: -DWITH_MAGICMIME
 
   if flag(Benchmark)
     Build-Depends: criterion