Update miniupnpc include paths

[Sticksman]

Fixes some include since we removed adding the miniupnpc lib to the path.

Fixes #99196

[Sticksman]

Looks like it doesn't build for other platforms, will take a look

[akien-mga]

The problem here is that unlike most C/C++ libraries, miniupnpc has a discrepancy between the includes it uses for building from source, and the includes it expects from other applications using its public headers.

The files in thirdparty/miniupnpc/src/ use includes like #include "miniupnpcstrings.h", and those includes are in thirdparty/miniupnpc/include/, which is added to the include path when using the builtin_miniupnpc=yes code path (default).

So Godot's own code that uses this builtin copy of miniupnpc also has thirdparty/miniupnpc/include/ as include path, and includes those headers without a miniupnpc/ prefix.

But the miniupnpc pkgconfig file forces a use relative to /usr/include, and expects the paths to the miniupnpc/miniupnpcstrings.h, which can't be true both when using our vendored copy (with no miniupnpc/ subfolder) and the system one.

We could move thirdparty/miniupnpc/include/ to thirdparty/miniupnpc/include/miniupnpc/ and add this as include path for building the miniupnpc C files, but add thirdparty/miniupnpc/include/ only as include path for building Godot files, but that's pretty messy.

IMO we should just partially revert that change, as miniupnpc's pkgconfig file is IMO incorrect.

[akien-mga]

CC @dustdfg

[Sticksman]

Thank you for the very detailed explanation! That all sounds incredibly annoying and a splitting headache. We should update the issue with this explanation.