Fix warning: "assert" redefined

Fix warning: "assert" redefined

Commit 21726a7 "Add assert() for debug assertions" caused build
warnings for many systems:

In file included from bedbug.c:6:
/home/wd/git/u-boot/work/include/bedbug/bedbug.h:24:1: warning: "assert" redefined
In file included from bedbug.c:3:
/home/wd/git/u-boot/work/include/common.h:144:1: warning: this is the location of the previous definition
In file included from cmd_bedbug.c:10:
/home/wd/git/u-boot/work/include/bedbug/bedbug.h:24:1: warning: "assert" redefined
In file included from cmd_bedbug.c:5:
/home/wd/git/u-boot/work/include/common.h:144:1: warning: this is the location of the previous definition

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>