mjit.c: fix declaration of ruby_null_device

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62277 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -204,7 +204,7 @@ static VALUE valid_class_serials;
 /* Ruby level interface module.  */
 VALUE rb_mMJIT;
 /* Portable /dev/null, defined in file.c */
-extern const char *ruby_null_device;
+extern const char ruby_null_device[];
 
 #ifdef _WIN32
 /* Linker option to enable libruby in the build directory. */