batman-adv: Switch to crc32 header for crc32c

The crc32c() function was moved in commit 8df36829 ("lib/crc32:
standardize on crc32c() name for Castagnoli CRC32") from linux/crc32c.h to
linux/crc32.h. The former is just an include file which redirects to the
latter.

Avoid the indirection by directly including the correct header file.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>