ARM: 8918/2: only build return_address() if needed

The system currently warns if the config conditions for
building return_address in arch/arm/kernel/return_address.c
are not met, leaving just an EXPORT_SYMBOL_GPL(return_address)
of a function defined to be 'static linline'.
This is a result of aeea3592 ("ARM: 8158/1: LLVMLinux: use static inline in ARM ftrace.h").

Since we're not going to build anything other than an exported
symbol for something that is already being defined to be an
inline-able return of NULL, just avoid building the code to
remove the following warning:

Fixes: aeea3592 ("ARM: 8158/1: LLVMLinux: use static inline in ARM ftrace.h")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>