[site] Fix favicon links in <head>

These relative links didn't work from pages other than index.html.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>
diff --git a/site/landing/layouts/partials/head.html b/site/landing/layouts/partials/head.html
index f2e0ee1..5b7e92c 100644
--- a/site/landing/layouts/partials/head.html
+++ b/site/landing/layouts/partials/head.html
@@ -14,10 +14,10 @@
     <meta property="twitter:description" content="{{ .Site.Params.description }}">
     <meta name="twitter:card" content="summary_large_image">
 
-    <link rel="apple-touch-icon" sizes="180x180" href="apple-touch-icon.png">
-    <link rel="icon" type="image/png" sizes="32x32" href="favicon-32x32.png">
-    <link rel="icon" type="image/png" sizes="16x16" href="favicon-16x16.png">
-    <link rel="shortcut icon" href="favicon.ico">
+    <link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">
+    <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png">
+    <link rel="icon" type="image/png" sizes="16x16" href="/favicon-16x16.png">
+    <link rel="shortcut icon" href="/favicon.ico">
     <meta name="msapplication-TileColor" content="#c27bcf">
     <meta name="theme-color" content="#ffffff">