FileSearch: Check isDirectory in the non-Windows code

c5fa470 made the extension check discard directories, but
only in the new code that currently only is used on Windows.
Let's add an equivalent check in the old code so that the
behavior is consistent across platforms.

Source/Core/Common/FileSearch.cpp

@@ -52,6 +52,8 @@ std::vector<std::string> DoFileSearch(const std::vector<std::string>& directorie
   return FileSearchWithTest(directories, recursive, [&](const File::FSTEntry& entry) {
     if (accept_all)
       return true;
+    if (entry.isDirectory)
+      return false;
     std::string name = entry.virtualName;
     std::transform(name.begin(), name.end(), name.begin(), ::tolower);
     return std::any_of(exts.begin(), exts.end(), [&](const std::string& ext) {