ARM: LPC32xx: clock.c: Missing header file

This patch fixes the compiler warnings regarding the EXPORT_SYMBOL usage

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Kevin Wells <kevin.wells@nxp.com>
Signed-off-by: Olof Johansson <olof@lixom.net>