fbdev: generated logo sources depend on scripts/pnmtologo

The generated logo sources are not automatically regenerated if
scripts/pnmtologo.c has changed. Add the missing dependency to fix this.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>