Include common.h in qsort.c to fix build warning

exports.h no longer includes common.h, which contains assert(). qsort.c
needs to be updated. This fixes this warning:

qsort.c: In function 'qsort':
qsort.c:30:3: warning: implicit declaration of function 'assert' [-Wimplicit-function-declaration]

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Tested-by: Heiko Schocher <hs@denx.de>