clang-tidy: don't use string compare

Found with readability-string-compare

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/futils.cpp

@@ -277,7 +277,7 @@ namespace Exiv2 {
     bool fileExists(const std::string& path, bool ct)
     {
         // special case: accept "-" (means stdin)
-        if (path.compare("-") == 0 || fileProtocol(path) != pFile) {
+        if (path == "-" || fileProtocol(path) != pFile) {
             return true;
         }