libsysprof: add instrument to record scheduler switching

This is to allow us to look at what processes were running when as a way
to get a high-level overview of usage.

src/libsysprof/meson.build

@@ -50,6 +50,7 @@ libsysprof_public_sources = [
   'sysprof-proxied-instrument.c',
   'sysprof-recording.c',
   'sysprof-sampler.c',
+  'sysprof-scheduler-details.c',
   'sysprof-spawnable.c',
   'sysprof-subprocess-output.c',
   'sysprof-symbol.c',
@@ -114,6 +115,7 @@ libsysprof_public_headers = [
   'sysprof-proxied-instrument.h',
   'sysprof-recording.h',
   'sysprof-sampler.h',
+  'sysprof-scheduler-details.h',
   'sysprof-spawnable.h',
   'sysprof-subprocess-output.h',
   'sysprof-symbol.h',