Always use 64-bit file offsets

Previously, OFD locking was broken on 32-bit, because it would somehow
end up using the 64-bit fcntl() function, but pass it 32-bit off_t
values in struct flock.  To fix this, tell libc to use always use
64-bit file offsets.

This is the same fix that was applied for a similar issue in GHC[1].

[1]: https://gitlab.haskell.org/ghc/ghc/-/commit/9853fc5e3556e733b56976b0a2fce9e82130a9ef

Fixes: haskellari#15

src-ofd/Lukko/OFD.hsc

@@ -33,6 +33,7 @@ module Lukko.OFD (
     hUnlock,
     ) where
 
+#define _FILE_OFFSET_BITS 64
 #define _GNU_SOURCE
 #include <unistd.h>