MDEV-9618 solaris sparc build fails on 10.1.

Compiler there is strict about the C/C++ call model
        mixing in function variable assumptions.
        Fixed by adding some 'extern "C"' and changing
        '?' operator with 'if'.

include/mysql/plugin_encryption.h

@@ -27,6 +27,10 @@
 
 #include <mysql/plugin.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define MariaDB_ENCRYPTION_INTERFACE_VERSION 0x0300
 
 /**
@@ -114,5 +118,9 @@ struct st_mariadb_encryption
   */
   unsigned int (*encrypted_length)(unsigned int slen, unsigned int key_id, unsigned int key_version);
 };
+
+#ifdef __cplusplus
+}
+#endif
 #endif