Fix comparison warning (fcitx#420)

This fix a comparison warning between int and size_t,
which e99498a causes.

src/modules/spell/spell-enchant.cpp

@@ -46,7 +46,7 @@ std::vector<std::string> SpellEnchant::hint(const std::string &language,
     std::vector<std::string> result;
     number = number > limit ? limit : number;
     result.reserve(number);
-    for (auto i = 0; i < number; i++) {
+    for (size_t i = 0; i < number; i++) {
         result.push_back(suggestions[i]);
     }