docs/manual/prerequisite.txt: add diffutils

The command `cmp` is used by `package/pkg-generic.mk` at commit
8623cc5deb5. This command is provided by `diffutils`, so it should be
installed on the host system.

Signed-off-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/prerequisite.txt

@@ -23,6 +23,7 @@ between distributions).
 ** +make+ (version 3.81 or any later)
 ** +binutils+
 ** +build-essential+ (only for Debian based systems)
+** +diffutils+
 ** +gcc+ (version 4.8 or any later)
 ** `g++` (version 4.8 or any later)
 ** +bash+