Bug#1039526: gcc-9-cross-mipsen: Update to autoconf2.69

[Bastian Germann]

Source: gcc-9-cross-mipsen
Version: 4+c2

Please update the autoconf2.64 build dependency to autoconf2.69 which is also 
implied by gcc-9-source.
It is of the last autoconf2.64 reverse dependencies, so updating contributes to 
dropping autoconf2.64 from the archive.