libc/wchar: remove unnecessary config

Signed-off-by: Jiuzhu Dong <dongjiuzhu1@xiaomi.com>

include/stdlib.h

@@ -204,13 +204,11 @@ FAR char *itoa(int val, FAR char *str, int base);
 
 /* Wide character operations */
 
-#ifdef CONFIG_LIBC_WCHAR
 int       mblen(FAR const char *s, size_t n);
 int       mbtowc(FAR wchar_t *pwc, FAR const char *s, size_t n);
 size_t    mbstowcs(FAR wchar_t *dst, FAR const char *src, size_t len);
 int       wctomb(FAR char *s, wchar_t wchar);
 size_t    wcstombs(FAR char *dst, FAR const wchar_t *src, size_t len);
-#endif
 
 /* Memory Management */