PDCurses: pdcscrn.c: Use #ifdef instead of #if CONFIG_SPEAKER

Building libpayload with the PDCurses backend the following warning
is shown.

    /src/coreboot/payloads/libpayload(master) $ make clean
    /src/coreboot/payloads/libpayload(master) $ make
    […]
        CC         curses/pdcurses-backend/pdcscrn.libcurses.o
    curses/pdcurses-backend/pdcscrn.c: In function 'PDC_scr_open':
    curses/pdcurses-backend/pdcscrn.c:75:5: warning: "CONFIG_SPEAKER" is not defined [-Wundef]
    […]

The GCC documentation states [1]

    In some contexts this shortcut is undesirable. The -Wundef option
    causes GCC to warn whenever it encounters an identifier which is
    not a macro in an ‘#if’.

and therefore use `#ifdef` [2] to silence this warning. No functional
change is done, as `CONFIG_SPEAKER` is assigned the value `Y` when
defined.

There was some discussion going on the list [3], but my points in there
turned out to be incorrect.

[1] http://gcc.gnu.org/onlinedocs/cpp/If.html
[2] http://gcc.gnu.org/onlinedocs/cpp/Ifdef.html
[3] http://www.coreboot.org/pipermail/coreboot/2013-March/075561.html

Change-Id: I8e9c9b5d01985b21ad05018986d614cf9bf2b439
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/2934
Reviewed-by: Nico Huber <nico.huber@secunet.com>
Tested-by: build bot (Jenkins)

payloads/libpayload/curses/pdcurses-backend/pdcscrn.c

@@ -72,7 +72,7 @@ int PDC_scr_open(int argc, char **argv)
     SP->lines = PDC_get_rows();
     SP->cols = PDC_get_columns();
 
-#if CONFIG_SPEAKER
+#ifdef CONFIG_SPEAKER
     SP->audible = TRUE;
 #endif