cgit.css: set form margins

According to the css2 spec, htmlforms have 1.12em top and bottom margins. That
doesn't play well with the placement of the search form, so lets force it to
use 0em margins.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.css

@@ -1,4 +1,4 @@
-body, table {
+body, table, form {
 	padding: 0em;
 	margin: 0em;
 }