crypto: crypto4xx - remove extern statement before function declaration

All function declarations are "extern" by default, there is no need to
specify it explicitly.

For C99 states in 6.2.2.5:
"If the declaration of an identifier for a function has no
storage-class specifier, its linkage is determined exactly
as if it were declared with the storage-class specifier
extern."

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>