libsysprof-capture: Move MappedRingBufferSource to libsysprof

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

src/libsysprof/meson.build

@@ -72,6 +72,7 @@ libsysprof_private_sources = [
   'binfile.c',
   'demangle.cpp',
   'elfparser.c',
+  'mapped-ring-buffer-source.c',
   'sysprof-flatpak.c',
   'sysprof-helpers.c',
   'sysprof-kallsyms.c',