#235 Update MSYS2 Building Instructions

[Greg King]

The new version of MSYS2's initial installer doesn't come with the command that we use to install the packages that support VICE. Therefore, this patch adds a command line to the "Windows-MinGW-GTK3-Howto.txt" instructions. That line installs the command that we need.

[compyx]

Applied in r38106. Also changed the line about altering $PATH to use .bash_profile and not .bashrc, as you mentioned on the mailing list.

Thanks!