Add missing INFO-DIR-SECTION to silence install-info(1)

--- docs/DevIL_manual.texi.orig	Sun Aug 28 19:21:00 2011
+++ docs/DevIL_manual.texi	Sun Aug 28 19:24:02 2011
@@ -13,6 +13,15 @@
 @c version.texi is generated automatically
 @include version.texi
 
+@ifinfo
+@format
+INFO-DIR-SECTION Libraries
+START-INFO-DIR-ENTRY
+* DevIL_manual: (devil_manual).         IL part of DevIL - handling images
+END-INFO-DIR-ENTRY
+@end format
+@end ifinfo
+
 @copying
 This is @code{DevIL @value{VERSION}} manual.
 Last update is from @value{UPDATED}.