ARC: hide unused function unw_hdr_alloc

As kernelci.org reports, this function is not used in
vdk_hs38_defconfig:

arch/arc/kernel/unwind.c:188:14: warning: 'unw_hdr_alloc' defined but not used [-Wunused-function]

Fixes: bc79c9a7 ("ARC: dw2 unwind: Reinstante unwinding out of modules")
Link: https://kernelci.org/build/id/5d1cae3f59b514300340c132/logs/


Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>