Add view port in html header in drupal 7

Add below function in template.php of your theme
replace emerachem with your theme name .


function emerachem_preprocess_html(&$vars) { 
  $viewport = array(
   '#tag' => 'meta',
   '#attributes' => array(
     'name' => 'viewport',
     'content' => 'width=device-width, initial-scale=1, maximum-scale=1',
   ),
  );
  drupal_add_html_head($viewport, 'viewport');
}

after adding this code clear chache