include: #undef crc32c

Some versions of linux-libc header files define a macro named 'crc32c'.
We want to use that name too, so #undef it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/btrfs.h

@@ -20,6 +20,7 @@
 #define mutex not_mutex
 #include <btrfs/ctree.h>
 // Repair the damage
+#undef crc32c
 #undef min
 #undef max
 #undef mutex