Correct Makefile rule for generating custom keymap

When building a custom keymap, after setting GENERATE_KEYMAP := 1 in
drivers/char/Makefile, the kernel build fails like this:

CC drivers/char/vt.o
make[2]: *** No rule to make target `drivers/char/%.map', needed by `drivers/char/defkeymap.c'. Stop.
make[1]: *** [drivers/char] Error 2
make: *** [drivers] Error 2

This was caused by commit af8b128719f5248e542036ea994610a29d0642a6, which
deleted a necessary colon from the Makefile rule that generates the keymap,
since that rule contains both a target and a target-pattern. The following
patch puts the colon back:

Signed-off-by: Maarten Bressers <mbres@gentoo.org>
Cc: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Cc: Ralf Baechle <ralf@linux-mips.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>