krb5: fix detecting channel binding feature

Use the already detected `gssapi/gssapi_krb5.h` MIT Kerberos header
to pull in `gssapi_ext.h`, which in turn sets `GSS_C_CHANNEL_BOUND_FLAG`
if supported. Channel binding is present in MIT Kerberos 1.19+.

Also:
- lib: de-duplicate GSS-API header includes.
- vauth: de-duplicate `urldata.h` includes.
- drop interim feature macro in favor of the native GSS one.

Assisted-by: Max Faxälv
Reported-by: Max Faxälv
Bug: https://github.com/curl/curl/pull/19164#issuecomment-3551687025
Follow-up to 8616e5aada #19164
Closes #19603
Closes #19760

lib/vauth/ntlm.c

@@ -35,7 +35,7 @@
 
 #define DEBUG_ME 0
 
-#include "../urldata.h"
+#include "vauth.h"
 #include "../sendf.h"
 #include "../curl_ntlm_core.h"
 #include "../curl_gethostname.h"
@@ -44,8 +44,6 @@
 #include "../rand.h"
 #include "../vtls/vtls.h"
 #include "../strdup.h"
-
-#include "vauth.h"
 #include "../curl_endian.h"
 
 /* NTLM buffer fixed size, large enough for long user + host + domain */