As preparation for dropping the GLib dependency from libsysprof-capture,
move the `GSource` which links a `MappedRingBuffer` to a `GMainContext`
from libsysprof-capture to libsysprof.
This requires adding one new piece of API to libsysprof-capture to check
whether the `MappedRingBuffer` is empty.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Helps: #40
