clocksource/drivers/ixp4xx: remove EXPORT_SYMBOL_GPL from ixp4xx_timer_setup()

ixp4xx_timer_setup is exported, and so can not be an __init function.
But it does not need to be exported as it is only called from one
in-kernel function, so just remove the EXPORT_SYMBOL_GPL() marking to
resolve the build warning.

This is fixed "properly" in commit 41929c9f
("clocksource/drivers/ixp4xx: Drop boardfile probe path") but that can
not be backported to older kernels as the reworking of the IXP4xx
codebase is not suitable for stable releases.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>