[ARM] fix sharp zaurus c-3000 compile failure without CONFIG_FB_PXA

This fixes compile problem when CONFIG_FB_PXA is not set.

  LD      .tmp_vmlinux1
arch/arm/mach-pxa/built-in.o(.text+0x1d74): In function
`spitz_get_hsync_len':
: undefined reference to `pxafb_get_hsync_time'
make: *** [.tmp_vmlinux1] Error 1
3.46user 0.46system 5.10 (0m5.106s) elapsed 77.01%CPU

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>