theme: adds padding to search-provider-icon

Adds 15px padding to all sides of provider icon to have padding
which seems equal to that of list-search-result-content. This aligns the
provider icon vertically with the search result content.

Padding is set to 15px as list-search-result-content has 12px padding
and the outer box (list-search-result) has 3px.

http://bugzilla.gnome.org/show_bug.cgi?id=695760

data/theme/gnome-shell.css

@@ -964,6 +964,10 @@ StScrollBar StButton#vhandle:active {
     text-align: center;
 }
 
+.search-provider-icon {
+    padding: 15px;
+}
+
 .app-folder-popup {
     -arrow-border-radius: 8px;
     -arrow-background-color: black;