[wwwdocs] Ensure the copyright footer always goes across the full page

[Gerald Pfeifer]

...without overlap.

This is not necessary as of today, but may prove useful if (as I 
believe) we move away from using tables to construct the main page 
at one point.  Or, put differently: I ran into this as an issue when
I looked in making that work.  And it's always a good safety net

Committed.

Gerald

Index: gcc.css
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc.css,v
retrieving revision 1.67
diff -u -r1.67 gcc.css
--- gcc.css     9 Sep 2018 21:18:17 -0000       1.67
+++ gcc.css     23 Sep 2018 16:40:58 -0000
@@ -74,6 +74,7 @@
 }
 
 div.copyright {
+  clear: both;
   font-size: smaller;
   background: #f2f2f9;
   border: 2px solid #3366cc;