perf annotate: Rename source_line_percent to source_line_samples

To better reflect the purpose of this struct, that is to hold
info about samples, its total number and is percentage.

Cc: Martin Liska <mliska@suse.cz>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-6bf8gwcl975uurl0ttpvtk69@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>