Fix Clang 19 deprecation failure

Fixes #3994

Clang 19 with libc++ started failing to compile because the default
implementation of `std::char_traits` was removed, making uses of
`std::char_traits<unsigned char>` invalid. This was more of used for
convenience before, but it should be roughly the same behavior with
`char`.

See relevant LLVM commits:

aeecef08c3

08a0faf4cd

src/DFA.h

@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include <sys/types.h> // for u_char
+#include <sys/types.h>
 #include <cassert>
 #include <map>
 #include <string>
@@ -67,7 +67,7 @@ protected:
     DFA_State* mark;
 };
 
-using DigestStr = std::basic_string<u_char>;
+using DigestStr = std::string;
 
 struct DFA_State_Cache_Stats {
     // Sum of all NFA states