libstdc++: Make permissions_are_testable function inline [PR90787]

This function should be inline, so that's it's not emitted in tests that
don't use it, to avoid undefined references to geteuid().

libstdc++-v3/ChangeLog:

	PR libstdc++/90787
	* testsuite/util/testsuite_fs.h (permissions_are_testable):
	Define as inline.

libstdc++-v3/testsuite/util/testsuite_fs.h

@@ -160,7 +160,7 @@ namespace __gnu_test
     path_type path;
   };
 
-  bool
+  inline bool
   permissions_are_testable(bool print_msg = true)
   {
     bool testable = false;