lpc32xx_loopback_set() is defined in linux/soc/nxp/lpc32xx-misc.h but
this is not included before the function definition.

arch/arm/mach-lpc32xx/serial.c:63:6: error: no previous prototype for 'lpc32xx_loopback_set'

Fixes: ffba29c9 ("serial: lpc32xx: allow compile testing")
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Link: https://lore.kernel.org/r/20230516153109.514251-7-arnd@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>