x86: Declare global_data pointer when it is used

Several files use the global_data pointer without declaring it. This works
because the declaration is currently a NOP. But still it is better to
fix this so that x86 lines up with other archs.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/interrupts.c b/arch/x86/cpu/interrupts.c
index 6dc74e3..e733bcb 100644
--- a/arch/x86/cpu/interrupts.c
+++ b/arch/x86/cpu/interrupts.c
@@ -37,6 +37,8 @@
 #include <asm/msr.h>
 #include <asm/u-boot-x86.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 #define DECLARE_INTERRUPT(x) \
 	".globl irq_"#x"\n" \
 	".hidden irq_"#x"\n" \