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_password_validation.h

@@ -26,6 +26,10 @@
 
 #include <mysql/plugin.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define MariaDB_PASSWORD_VALIDATION_INTERFACE_VERSION 0x0100
 
 /**
@@ -41,5 +45,10 @@ struct st_mariadb_password_validation
   int (*validate_password)(MYSQL_LEX_STRING *username,
                            MYSQL_LEX_STRING *password);
 };
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif