Remove cgpt app-specific symbols from libvboot_host.a

Three symbols used by the standalone cgpt executable were being referenced
in the files used to create the external libvboot_host.a needed by non-vboot
userspace applications.

This cleans things up so those symbols don't have to be explictly defined
by other repos just to link with that library.

BUG=chromium:318536
BRANCH=ToT
TEST=manual

No new functionality, just code cleanup. Tested with

  make runtests runfutiltests runlongtests

Change-Id: Ibc77fb9800c89d7109ebf38d4d6729f52665722f
Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/205667
Reviewed-by: Randall Spangler <rspangler@chromium.org>
diff --git a/cgpt/cmd_show.c b/cgpt/cmd_show.c
index 1b31f39..df4cba1 100644
--- a/cgpt/cmd_show.c
+++ b/cgpt/cmd_show.c
@@ -10,6 +10,8 @@
 #include "cgpt.h"
 #include "vboot_host.h"
 
+extern const char* progname;
+
 static void Usage(void)
 {
   printf("\nUsage: %s show [OPTIONS] DRIVE\n\n"