Note that iswalnum etc functions defined in wctype.h, and cwchar uses them.
So include wctype.h to fix libcxx build break.
Change-Id: I0f6b402d817f0927729800bdb8b9fce15e76ec3d
Signed-off-by: liuhaitao <liuhaitao@xiaomi.com>
When a build does not want to use wide or multibyte char
CONFIG_LIBC_WCHAR is not set. Therefore we should to only
export the wc/mb functions when defined.
Regardless of the stat of CONFIG_LIBC_WCHAR the non mb/wc
definitions such as mbstate_t, wint_t, wctype_t need to be
exported.
