mjit.c: fix -Wdiscarded-qualifiers warning

../mjit.c: In function ‘system_tmpdir’:
../mjit.c:1231:9: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  tmpdir = "/tmp";
         ^
../mjit.c: At top level:

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62288 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -1228,7 +1228,7 @@ system_tmpdir(void)
 #endif
     if (!(tmpdir = getenv("TMPDIR")) &&
 	!(tmpdir = getenv("TMP"))) {
-	tmpdir = "/tmp";
+        return get_string("/tmp");
     }
     return get_string(tmpdir);
 }