common: move BUILD_BUG_ON define to common.h

see discussion also here:
http://patchwork.ozlabs.org/patch/75309/



Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Holger Brunck <holger.brunck@keymile.com>