gdb: rename source_styling_changed observer

gdb: rename source_styling_changed observer

In a later commit I plan to add disassembler styling.  In the same way
that we have a source_styling_changed observer I would need to add a
disassembler_styling_changed observer.

However, currently, these observers would only be notified from
cli-style.c:set_style_enabled, and observed in tui-winsource.c,
tui_source_window::style_changed, as a result, having two observers
seems unnecessary right now, so, in this commit, I plan to rename
source_styling_changed to just styling_changed, then, in the later
commit, when disassembler styling is added, I can use the same
observer for both source styling, and disassembler styling.

There should be no user visible changes after this commit.