Remove libstroke-related commands from the "Void Linux" section of INSTALL.md

[jdeokkim]

fvwmorg/fvwm3#165

From mikeandmore/fvwm3@e0dad09:

Remove support for libstroke

Although mouse gestures via libstroke were a nice experiment, its use
never really took off, in all but a few configurations.

Given the library itself is no longer maintained, some Linux
distributions have stopped shipping it altogether. Although FVWM3 can
be told to not compile with libstroke support, removing the code
altogether makes sense.