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/lib/relocate.c b/arch/x86/lib/relocate.c
index f178db9..21982db 100644
--- a/arch/x86/lib/relocate.c
+++ b/arch/x86/lib/relocate.c
@@ -39,6 +39,8 @@
 #include <asm/sections.h>
 #include <elf.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 int copy_uboot_to_ram(void)
 {
 	size_t len = (size_t)&__data_end - (size_t)&__text_start;