[IA64] Guard elfcorehdr_addr with #if CONFIG_PROC_FS

Access to elfcorehdr_addr needs to be guarded by #if CONFIG_PROC_FS
as well as the existing #if guards.

Fixes the following build problem:

arch/ia64/hp/common/built-in.o: In function
`sba_init':arch/ia64/hp/common/sba_iommu.c:2043: undefined reference to `elfcorehdr_addr'
:arch/ia64/hp/common/sba_iommu.c:2043: undefined reference to `elfcorehdr_addr'

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>