Try to fix error on solaris

Add vm_core.h before builtin.h to pack.c for VM_CORE_H_EC_DEFINED.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/solaris11s-sunc/ruby-master/log/20191112T102505Z.fail.html.gz
```
compiling dmyenc.c
linking miniruby
Undefined			first referenced
 symbol  			    in file
rb_vm_lvar_exposed                  pack.o
ld: fatal: symbol referencing errors
gmake: *** [miniruby] Error 2
```

pack.c

@@ -15,6 +15,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <float.h>
+#include "vm_core.h"
 #include "builtin.h"
 
 /*