kgdb: depends on VT

kgdb: depends on VT

With VT=n, the kernel build fails with:

  drivers/built-in.o: In function `kgdboc_pre_exp_handler':
  kgdboc.c:(.text+0x7b5aa): undefined reference to `fg_console'
  kgdboc.c:(.text+0x7b5ce): undefined reference to `vc_cons'
  kgdboc.c:(.text+0x7b5d5): undefined reference to `vc_cons'

kgdboc.o is built when KGDB_SERIAL_CONSOLE is set.  So make
KGDB_SERIAL_CONSOLE depend on HW_CONSOLE which includes those symbols.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Slaby <[email protected]>
Reported-by: "Jim Davis" <[email protected]>
Acked-by: Jason Wessel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>