core: fix compile error with CFG_CORE_WORKAROUND_SPECTRE_BP_SEC=n
core: fix compile error with CFG_CORE_WORKAROUND_SPECTRE_BP_SEC=n

Fixes compile error:
  CC      ../out-os-qemu/core/arch/arm/mm/tee_mm.o
core/arch/arm/kernel/thread.c:984:16: error: ‘select_vector’ defined but not used [-Werror=unused-function]
 static vaddr_t select_vector(vaddr_t a)
                ^~~~~~~~~~~~~
cc1: all warnings being treated as errors

when compiled with CFG_CORE_WORKAROUND_SPECTRE_BP_SEC=n

Reviewed-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Signed-off-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os