manual: prerequisite.txt: add texinfo to the mandatory package list

Using the internal Buildroot toolchain backend, makeinfo, whom belongs to
the texinfo package, is required to build gcc and gdb.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/prerequisite.txt

@@ -42,6 +42,7 @@ Mandatory packages
 ** +python+ (version 2.6 or 2.7)
 ** +unzip+
 ** +rsync+
+** +texinfo+ (required for internal Buildroot toolchain backend)
 
 * Source fetching tools:
 ** +wget+