[build] Mark GDB stub as forbidden for UEFI Secure Boot

Enabling the GDB debugger functionality would provide an immediate and
trivial Secure Boot exploit.  Mark all GDB-related files as explicitly
forbidden for UEFI Secure Boot.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/commands/gdbstub_cmd.c

@@ -22,6 +22,7 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( FORBIDDEN );
 
 #include <stdio.h>
 #include <errno.h>