libcrc32c: Fix "crc32c undefined" compilation error

The latest shash changes leave crc32c undefined:

[...]
Building modules, stage 2.
  MODPOST 1381 modules
  ERROR: "crc32c" [net/sctp/sctp.ko] undefined!
  ERROR: "crc32c" [net/ipv4/netfilter/nf_nat_proto_sctp.ko] undefined!

Adding EXPORT_SYMBOL(crc32c) to lib/libcrc32c.c fixes the compile error.
This patch has been compile-tested only.

Signed-off-by: Adrian-Ken Rueegsegger <rueegsegger@swiss-it.ch>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>