kconfig: fix make xconfig when gettext is missing

The C-based config programs are properly guarded from a missing (or,
currently, external) libintl.h by the HOST_EXTRACFLAGS check, but
this does not help the C++-based qconf.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>