Call a function in the MySQL configure test to make sure it links.

This fixes a problem where the configure test may produce a false
positive on iOS if a copy of libmysql for OS X is located, due to the
fact that linking to a dynamic library of a different architecture than
expected is not (yet) an error.

Change-Id: Id41765f49e31d9c9c3becc3436ff5a69b160b8e0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

config.tests/unix/mysql/mysql.cpp

@@ -35,5 +35,6 @@
 
 int main(int, char **)
 {
+    mysql_get_client_version();
     return 0;
 }