Use typedef names in evp.h.

They're all forward-declared. There's no need to use the struct names.

Change-Id: I435ae2f5971128f08c730317ca644d97239f3b54
Reviewed-on: https://boringssl-review.googlesource.com/5260
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/evp.h

diff --git a/include/openssl/evp.h b/include/openssl/evp.h
index 249bb3f..55f360a 100644
--- a/include/openssl/evp.h
+++ b/include/openssl/evp.h
@@ -681,10 +681,10 @@
 
   union {
     char *ptr;
-    struct rsa_st *rsa; /* RSA */
-    struct dsa_st *dsa; /* DSA */
-    struct dh_st *dh; /* DH */
-    struct ec_key_st *ec; /* ECC */
+    RSA *rsa;
+    DSA *dsa;
+    DH *dh;
+    EC_KEY *ec;
   } pkey;
 
   /* ameth contains a pointer to a method table that contains many ASN.1