gdb/tui: make m_horizontal_offset private

gdb/tui: make m_horizontal_offset private

I noticed that tui_source_window_base::m_horizontal_offset was
protected, but could be made private, so lets do that.

This makes more sense in the context of a later commit where I plan to
add another member variable that is similar to m_horizontal_offset.
The new member variable could also be private.

So I had to choose, place the new member variable next to
m_horizontal_offset making it protected, but grouping similar
variables together, or make m_horizontal_offset private, and then add
the new variable as private too.

I chose to make m_horizontal_offset private, which is this commit.

There should be no user visible changes after this commit.