[libc][obvious] Removed mbstate_t include (llvm#152046)

Internal function does not use the external mbstate_t but still had the
include for it.

Co-authored-by: Sriya Pratipati <sriyap@google.com>

Author: sribee8

libc/src/__support/wchar/mbrtowc.cpp

@@ -8,7 +8,6 @@
 
 #include "src/__support/wchar/mbrtowc.h"
 #include "hdr/errno_macros.h"
-#include "hdr/types/mbstate_t.h"
 #include "hdr/types/size_t.h"
 #include "hdr/types/wchar_t.h"
 #include "src/__support/common.h"