string.c: fix up r60748

An #ifdef was missing in r60748 and build broke on systems without
crypt_r().

https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11s/ruby-trunk/log/20171112T162503Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60749 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -8889,7 +8889,9 @@ rb_str_crypt(VALUE str, VALUE salt)
 	rb_sys_fail("crypt");
     }
     result = rb_str_new_cstr(res);
+#ifdef HAVE_CRYPT_R
     ALLOCV_END(databuf);
+#endif
     FL_SET_RAW(result, OBJ_TAINTED_RAW(str) | OBJ_TAINTED_RAW(salt));
     return result;
 }