emulators/ppsspp: unbreak with ffmpeg 4.0

Core/HLE/sceMpeg.cpp:890:27: error: use of undeclared identifier 'FF_INPUT_BUFFER_PADDING_SIZE'

u8* str = new u8[size + FF_INPUT_BUFFER_PADDING_SIZE];
                        ^

Core/HLE/sceMpeg.cpp:892:25: error: use of undeclared identifier 'FF_INPUT_BUFFER_PADDING_SIZE'

memset(str + size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
                      ^

Core/HLE/sceMpeg.cpp:893:11: error: use of undeclared identifier 'FF_INPUT_BUFFER_PADDING_SIZE'

size += FF_INPUT_BUFFER_PADDING_SIZE;
        ^

PR: 227726
Obtained from: upstream