ARMV7: Vexpress: Fix build error

Fix ca9x4_ct_vxp build error

Configuring for ca9x4_ct_vxp board...
board/armltd/vexpress/libvexpress.o: In function `udelay':
u-boot.git/board/armltd/vexpress/ca9x4_ct_vxp.c:161: multiple
definition of `udelay'
lib/libgeneric.o:u-boot.git/lib/time.c:34: first defined here

lib/libgeneric.o: In function `udelay':

zlib.c:(.text+0x1ee8): undefined reference to `__udelay'

Signed-of-by: Dirk Behme <dirk.behme@googlemail.com>