Add a right margin to the search bar

This creates some space between the search bar and the
"header-browse-source" endpoint where you often see a "Repositories"
link.

Screenshots: https://imgur.com/a/19eLQme
Change-Id: I54cc7540dd56628eeb3183da6195cd44f9ba536f
(cherry picked from commit 174850ddb44234ff986c04e949ea60856bdb062e)

polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html

diff --git a/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html b/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
index fe8384e..d29858e 100644
--- a/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
+++ b/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
@@ -90,7 +90,7 @@
       }
       gr-smart-search {
         flex-grow: 1;
-        margin-left: var(--spacing-m);
+        margin: 0 var(--spacing-m);
         max-width: 500px;
       }
       gr-dropdown,