x86: Add missing DECLARE_GLOBAL_DATA_PTR for mtrr.c

arch/x86/cpu/mtrr.c has access to the U-Boot global data thus
DECLARE_GLOBAL_DATA_PTR is needed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>