docs: fix copyright years in PDF version of the manual.

[Ralf Wildenhues]

libtool.pdf was showing only the current year in the copyright
statement.  This patch fixes it, applied to master.

Cheers,
Ralf

    docs: fix copyright years in PDF version of the manual.
    
    * doc/libtool.texi: Fix copyright years.

diff --git a/doc/libtool.texi b/doc/libtool.texi
index 070bc9b..3c7ff49 100644
--- a/doc/libtool.texi
+++ b/doc/libtool.texi
@@ -53,7 +53,7 @@ identical to this one except for the removal of this paragraph
 
 @page
 @vskip 0pt plus 1filll
-Copyright @copyright{} 2011 Free Software Foundation, Inc.
+Copyright @copyright{} 1996-2011 Free Software Foundation, Inc.
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3