libpayload: Fix build when both USB and PS/2 keyboard support is disabled

libpayload: Fix build when both USB and PS/2 keyboard support is disabled

libpayload uses -Werror for some reason right now, and the
variable 'c' in curses_getchar is only used if CONFIG_USB_HID
or CONFIG_PC_KEYBOARD is defined, giving an unused variable
warning that gets promoted to an error.
So wrap the variable declaration around appropriate #ifdef's

Signed-off-by: Mart Raudsepp <leio@gentoo.org>
Acked-by: Ulf Jordan <jordan@chalmers.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3957 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1