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>

arch/x86/cpu/mtrr.c

@@ -17,6 +17,8 @@
 #include <asm/msr.h>
 #include <asm/mtrr.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 /* Prepare to adjust MTRRs */
 void mtrr_open(struct mtrr_state *state)
 {