Fix type of ptr field.

Since "ptr" is used to handle arbitrary other types it should be
void *.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/include/internal/evp_int.h

@@ -356,7 +356,7 @@ struct evp_pkey_st {
     const EVP_PKEY_ASN1_METHOD *ameth;
     ENGINE *engine;
     union {
-        char *ptr;
+        void *ptr;
 # ifndef OPENSSL_NO_RSA
         struct rsa_st *rsa;     /* RSA */
 # endif