Fix wchar.h circular include: define types before stdio.h

relibc's wchar.h included <stdio.h> before defining wint_t and
mbstate_t. The circular chain wchar.h → stdio.h → inttypes.h →
wchar.h caused gnulib's wchar.h wrapper (used by m4, bison, etc.)
to see 'unknown type name wint_t' and 'unknown type name mbstate_t'.

Fix: Move stdio.h and time.h from sys_includes (which cbindgen
emits before type definitions) into after_includes, after wchar_t,
wint_t, and mbstate_t are defined. Also define mbstate_t manually
in after_includes with a guard, and exclude it from cbindgen export
to prevent duplicate definitions.

src/header/wchar/cbindgen.toml

@@ -1,8 +1,4 @@
-sys_includes = [
-    "stdio.h",
-    "time.h",
-    "features.h",
-]
+sys_includes = ["features.h"]
 after_includes = """
 // int32_t, uint32_t, WCHAR_MIN, WCHAR_MAX
 #include <stdint.h>
@@ -25,6 +21,21 @@ after_includes = """
     typedef __WINT_TYPE__ wint_t;
 #endif // _WINT_T
 
+// Define mbstate_t before headers that may transitively re-include wchar.h.
+// The circular chain wchar.h → stdio.h → inttypes.h → wchar.h (and also
+// stdlib.h → wchar.h) would otherwise cause wint_t and mbstate_t to be
+// undefined in gnulib's wchar.h wrapper, breaking m4, bison, and other
+// gnulib-based packages.
+#ifndef _RELIBC_MBSTATE_T
+#define _RELIBC_MBSTATE_T
+typedef struct {} mbstate_t;
+#endif
+
+// Now include headers that depend on wchar_t/wint_t/mbstate_t.
+// These must come after the type definitions to break circular includes.
+#include <stdio.h>
+#include <time.h>
+
 // NULL, size_t, must come after wchar_t and wint_t
 #define __need_size_t
 #define __need_NULL
@@ -38,6 +49,9 @@ style = "Type"
 no_includes = true
 cpp_compat = true
 
+[export]
+exclude = ["mbstate_t"]
+
 [export.rename]
 "tm" = "struct tm"