docs: Add bc to required tools

We check for bc under required packages.  It should be listed as such in the
docs.

Signed-off-by: Marcus Hoffmann <m.hoffmann@cartelsol.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/prerequisite.txt

@@ -36,6 +36,7 @@ between distributions).
 ** +unzip+
 ** +rsync+
 ** +file+ (must be in +/usr/bin/file+)
+** +bc+
 
 * Source fetching tools:
 ** +wget+