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: cerbero#164 (closed)