Set default timezone during install

Installing with a recent PHP (tested with 5.6.25) leads to a system
warning about missing the default timezone setting that blocks the
installation. Since we select the timezone in the first install step, we
can use that before continuing with the second.

Fixes #21957

Signed-off-by: Damien Regad <dregad@mantisbt.org>

admin/install.php

@@ -338,7 +338,9 @@ function print_test( $p_test_description, $p_result, $p_hard_fail = true, $p_mes
 
 # got database information, check and install
 if( 2 == $t_install_state ) {
-	?>
+    # By now user has picked a timezone, ensure it is set
+    date_default_timezone_set( $f_timezone );
+?>
 
 <!-- Checking DB support-->
 <?php