Rename "lines" parameter in source-cache.h

Rename "lines" parameter in source-cache.h

A compile in the TUI somehow had "lines" defined as a macro.  This
caused a compile error when including source-cache.h after whatever
header did that.  I tracked this down to a #define in
/usr/include/term.h, so I just changed source-cache.h to avoid the
clash.

gdb/ChangeLog
2019-01-22  Tom Tromey  <tom@tromey.com>

	* source-cache.h (class source_cache) <get_source_lines,
	get_plain_source_lines, extract_lines>: Rename "lines" parameter.