amd/addrlib: mark returnCode as MAYBE_UNUSED in ElemGetExportNorm

amd/addrlib: mark returnCode as MAYBE_UNUSED in ElemGetExportNorm

Only used, when asserts are enabled.

Fixes an unused-but-set-variable warning with GCC 8:
 ../../../src/amd/addrlib/addrinterface.cpp: In function 'int ElemGetExportNorm(ADDR_HANDLE, const ELEM_GETEXPORTNORM_INPUT*)':
 ../../../src/amd/addrlib/addrinterface.cpp:835:23: warning: variable 'returnCode' set but not used [-Wunused-but-set-variable]
      ADDR_E_RETURNCODE returnCode = ADDR_OK;
                        ^~~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>