libpayload: Fix const warnings in keyname() and termname()

The keyname() and termname() functions were creating a whole lot of warnings of
the style

curses/PDCurses-3.4/pdcurses/keyname.c:41:9: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]

This patch fixes them.

Change-Id: Iae3c4e5201b48c2d2033cac48577e0462a34f309
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/2905
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

payloads/libpayload/curses/PDCurses-3.4/pdcurses/termattr.c

@@ -135,7 +135,7 @@ attr_t term_attrs(void)
            WA_RIGHT | WA_UNDERLINE;
 }
 
-char *termname(void)
+const char *termname(void)
 {
     PDC_LOG(("termname() - called\n"));