gh-36883: `src/sage/modular/arithgroup/farey.cpp`: Fix `#include` for `farey_symbol.h`
    
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This reverts one change of #36754, which broke building sagemath-
standard as reported in
#36754 (comment)

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URL: #36883
Reported by: Matthias Köppe
Reviewer(s):

Author: Release Manager

src/sage/modular/arithgroup/farey.cpp

@@ -29,7 +29,7 @@
 #include <Python.h>
 
 #include "farey.hpp"
-#include "farey_symbol.h"
+#include "sage/modular/arithgroup/farey_symbol.h"
 
 
 using namespace std;