export rb_eRactorIsolationError for MJIT

https://ci.appveyor.com/project/ruby/ruby/builds/36942168/job/7ugrpk0pndoly9wp
```
_ruby_mjit_p11920u0.c
C:\Users\appveyor\AppData\Local\Temp\1/_ruby_mjit_p11920u0.c(14) : warning C4005: 'GET_SELF' : macro redefinition
        c:\projects\ruby\vm_insnhelper.h(111) : see previous definition of 'GET_SELF'
   Creating library C:\Users\appveyor\AppData\Local\Temp\1/_ruby_mjit_p11920u0.lib and object C:\Users\appveyor\AppData\Local\Temp\1/_ruby_mjit_p11920u0.exp
_ruby_mjit_p11920u0.obj : error LNK2001: unresolved external symbol rb_eRactorIsolationError
C:\Users\appveyor\AppData\Local\Temp\1/_ruby_mjit_p11920u0.so : fatal error LNK1120: 1 unresolved externals
```

vm_core.h

@@ -2010,8 +2010,9 @@ extern void rb_reset_coverages(void);
 
 void rb_postponed_job_flush(rb_vm_t *vm);
 
-extern VALUE rb_eRactorUnsafeError; // ractor.c
-extern VALUE rb_eRactorIsolationError;
+// ractor.c
+RUBY_EXTERN VALUE rb_eRactorUnsafeError;
+RUBY_EXTERN VALUE rb_eRactorIsolationError;
 
 RUBY_SYMBOL_EXPORT_END