fix: add wchar_t definition to stddef.h (C mode only, C++ has it as keyword)

POSIX requires stddef.h to define wchar_t. Previously it was missing,
which broke inttypes.h (wcstoimax/strtoimax use wchar_t). Fixed by
adding  guarded by  since
C++ has wchar_t as a built-in keyword.

include/stddef.h

@@ -11,6 +11,10 @@ typedef __PTRDIFF_TYPE__ ptrdiff_t;
 
 typedef long unsigned int size_t;
 
+#ifndef __cplusplus
+typedef int wchar_t;
+#endif
+
 typedef struct { long long __ll; long double __ld; } max_align_t;
 
 #define offsetof(type, member) __builtin_offsetof(type, member)