common/file_util: Remove unnecessary c_str() calls

The file stream open functions have supported std::string overloads
since C++11, so we don't need to use c_str() here. Same behavior, less
code.

src/common/file_util.h

@@ -286,8 +286,8 @@ private:
 template <typename T>
 void OpenFStream(T& fstream, const std::string& filename, std::ios_base::openmode openmode) {
 #ifdef _MSC_VER
-    fstream.open(Common::UTF8ToUTF16W(filename).c_str(), openmode);
+    fstream.open(Common::UTF8ToUTF16W(filename), openmode);
 #else
-    fstream.open(filename.c_str(), openmode);
+    fstream.open(filename, openmode);
 #endif
 }