crucible: Try repairing a build failure around swap macro

Gentoo-Bug: https://bugs.gentoo.org/670606
Fixes: https://github.com/Zygo/bees/issues/85
Suggested-by: Zygo Blaxell <bees@furryterror.org>
Signed-off-by: Kai Krakow <kai@kaishome.de>

include/crucible/btrfs.h

@@ -23,6 +23,7 @@
 #undef min
 #undef max
 #undef mutex
+#undef swap
 
 #ifndef BTRFS_FIRST_FREE_OBJECTID