Documentation/Changes: perl is needed to build the kernel

Perl is used on the kernel Makefile to generate documentation, firmwares
in c source form, sources, graphs, and some headers and this fact is
undocumented.

[akpm@linux-foundation.org: 80-columns, please]
Signed-off-by: Jose Luis Perez Diez <jluis@escomposlinux.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>