don't forward-declare SSL types any more

our current project structure precludes the clash between some indirect
include of ssl.h and our definition of 'S' (or 'M', i don't remember)
that happened on some system, so there is no need to avoid including it
any more.

this avoids complaints from some more picky compilers, as re-defining
typedefs (even to the same thing) is illegal before C11.

src/socket.h

@@ -30,9 +30,7 @@
 #endif
 
 #ifdef HAVE_LIBSSL
-typedef struct ssl_st SSL;
-typedef struct ssl_ctx_st SSL_CTX;
-typedef struct stack_st _STACK;
+# include <openssl/ssl.h>
 
 enum {
 	SSLv3 = 2,