fit : avoid including llama-ext.h in fit.h (#24506)

src/llama-ext.h

@@ -2,6 +2,7 @@
 
 // this is a staging header for new llama.cpp API
 // breaking changes and C++ are allowed. everything here should be considered WIP
+// try as much as possible to not include this header in the rest of the codebase
 
 #include "llama.h"