[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/include/bits/gdbmach.h

@@ -10,6 +10,8 @@
  *
  */
 
+FILE_SECBOOT ( FORBIDDEN );
+
 #include <stdint.h>
 
 typedef unsigned long gdbreg_t;

src/include/ipxe/gdbserial.h

@@ -8,6 +8,7 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( FORBIDDEN );
 
 struct gdb_transport;
 

src/include/ipxe/gdbstub.h

@@ -8,6 +8,7 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( FORBIDDEN );
 
 #include <stdint.h>
 #include <ipxe/tables.h>

src/include/ipxe/gdbudp.h

@@ -8,6 +8,7 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( FORBIDDEN );
 
 struct sockaddr_in;
 struct gdb_transport;