add string.h

src/header/wctype/cbindgen.toml

-sys_includes = ["stddef.h", "stdint.h", "time.h", "stdio.h" ]
+sys_includes = ["stddef.h", "stdint.h", "time.h", "stdio.h", "string.h" ]
 include_guard = "_RELIBC_WCTYPE_H"
 header = "#include <bits/wctype.h>"
 language = "C"