crypto: user - fix potential warnings in cryptouser.h

Function definitions in headers are usually marked as 'static inline'.

Since 'inline' is missing for crypto_reportstat(), if it were not
referenced from a .c file that includes this header, it would produce
a warning.

Also, 'struct crypto_user_alg' is not declared in this header.

I included <linux/crytouser.h> instead of adding the forward declaration
as suggested [1].

Detected by compile-testing this header as a standalone unit:

./include/crypto/internal/cryptouser.h:6:44: warning: ‘struct crypto_user_alg’ declared inside parameter list will not be visible outside of this definition or declaration
 struct crypto_alg *crypto_alg_match(struct crypto_user_alg *p, int exact);
                                            ^~~~~~~~~~~~~~~
./include/crypto/internal/cryptouser.h:11:12: warning: ‘crypto_reportstat’ defined but not used [-Wunused-function]
 static int crypto_reportstat(struct sk_buff *in_skb, struct nlmsghdr *in_nlh, struct nlattr **attrs)
            ^~~~~~~~~~~~~~~~~

[1] https://lkml.org/lkml/2019/6/13/1121



Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>