Issue #10914: Initialize correctly the filesystem codec when creating a new
subinterpreter to fix a bootstrap issue with codecs implemented in Python, as
the ISO-8859-15 codec.

Add fscodec_initialized attribute to the PyInterpreterState structure.
diff --git a/Include/pystate.h b/Include/pystate.h
index a894bb4..96a70b9 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -31,6 +31,7 @@
     PyObject *codec_search_cache;
     PyObject *codec_error_registry;
     int codecs_initialized;
+    int fscodec_initialized;
 
 #ifdef HAVE_DLOPEN
     int dlopenflags;