openssl.h: adjust the rsa/dsa includes

... to work when built without DSA support.

Closes #36

src/openssl.h

@@ -39,6 +39,10 @@
 
 #include <openssl/opensslconf.h>
 #include <openssl/sha.h>
+#include <openssl/rsa.h>
+#ifndef OPENSSL_NO_DSA
+#include <openssl/dsa.h>
+#endif
 #ifndef OPENSSL_NO_MD5
 #include <openssl/md5.h>
 #endif