rtmp: Fix crash inside free() with MSVC on Windows

rtmp: Fix crash inside free() with MSVC on Windows

librtmp is always built with MinGW in Cerbero, so if the plugin is
built with MSVC and it frees memory allocated by librtmp, that leads
to a crash since the CRT used by MinGW and MSVC are different.

This is fixed in master by switching to a newer GCC toolchain which
has been configured to use the same CRT as newer versions of Visual
Studio, so there's no cross-CRT memory alloc/free issues.

See: https://gitlab.freedesktop.org/gstreamer/cerbero/issues/164