Bump to alitrack/mman-win32@2d1c576e

Changes: alitrack/mman-win32@dbba5db...2d1c576

  * alitrack/mman-win32@2d1c576: Merge pull request dotnet#12 from constantined/patch-1
  * alitrack/mman-win32@fd14269: Merge pull request dotnet#16 from ZachCook/patch-1
  * alitrack/mman-win32@6556d4f: Support MAP_FIXED using MapViewOfFileEx
  * alitrack/mman-win32@9f115ad: Merge pull request dotnet#15 from roydmerkel/master
  * alitrack/mman-win32@3874169: Added --bindir install option to configure so we can specify the bindir parameter.
  * alitrack/mman-win32@c6cc38f: Define bindir, libdir and incdir after custom PREFIX was set

An unexpected oddity: we previously used witwall/mman-win32, but
<https://github.com/witwall/mman-win32> is now an HTTP 301 (redirect)
to <https://github.com/alitrack/mman-win32>.

.gitmodules

@@ -20,7 +20,7 @@
     branch = master
 [submodule "external/mman-win32"]
     path = external/mman-win32
-    url = https://github.com/witwall/mman-win32.git
+    url = https://github.com/alitrack/mman-win32.git
     branch = master
 [submodule "external/nrefactory"]
     path = external/nrefactory