Rename GPT_HEADER_SIGNATURE to avoid conflict

The current name conflicts with the Chrome OS verified boot library, which
prevents it being built. That library uses a string whereas U-Boot uses a
64-bit hex value. Rename this in U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/part_efi.h b/include/part_efi.h
index 8525770..7170b61 100644
--- a/include/part_efi.h
+++ b/include/part_efi.h
@@ -24,7 +24,7 @@
 #define EFI_PMBR_OSTYPE_EFI 0xEF
 #define EFI_PMBR_OSTYPE_EFI_GPT 0xEE
 
-#define GPT_HEADER_SIGNATURE 0x5452415020494645ULL
+#define GPT_HEADER_SIGNATURE_UBOOT 0x5452415020494645ULL
 #define GPT_HEADER_REVISION_V1 0x00010000
 #define GPT_PRIMARY_PARTITION_TABLE_LBA 1ULL
 #define GPT_ENTRY_NUMBERS		CONFIG_EFI_PARTITION_ENTRIES_NUMBERS