Allow to compile CPP-file alone

Only for testing the compilation of this file.

[part of] Rework buffer into C++ class
Cherry-picked from: jellyfin@6d3a5c7

Author: JustAMan

src/SubtitleOctopus.cpp

@@ -351,4 +351,6 @@ class SubtitleOctopus {
 
 int main(int argc, char *argv[]) { return 0; }
 
+#ifdef __EMSCRIPTEN__
 #include "./SubOctpInterface.cpp"
+#endif