Make vboot_version extern in header

When enabling `-fno-common` in Fuchsia, we get a bunch of linker errors
when linking futility:

ld.lld: error: duplicate symbol: vboot_version
>>> defined at futility.h:43 (../../third_party/vboot_reference/futility/futility.h:43)
>>>            host_x63-asan_no_detect_leaks/obj/third_party/vboot_reference/futility/futility.cmd_bdb.c.o:(vboot_version)
>>> defined at futility.h:43 (../../third_party/vboot_reference/futility/futility.h:43)
>>>            host_x64-asan_no_detect_leaks/obj/third_party/vboot_reference/futility/futility.cmd_create.c.o:(.bss.vboot_version+0x0)

and think this is because -fno-common places vboot_version for
unitialized global variables in the BSS section of each object file.
Making it extern instead resolves each reference to its definition in

Change-Id: Ibc0ea747918569d0c854e7d2ee838e7537442eee
diff --git a/futility/futility.h b/futility/futility.h
index c815003..6a593fb 100644
--- a/futility/futility.h
+++ b/futility/futility.h
@@ -40,7 +40,7 @@
 /* What's our preferred API & data format? */
-enum vboot_version vboot_version;
+extern enum vboot_version vboot_version;
 /* Here's a structure to define the commands that futility implements. */
 struct futil_cmd_t {