Remove signal-source headers from public API

Those headers implement a platform-specific mechanism. They are never
used by components directly.

This patch also cleans up a few other remaining platform-specific
artifact such as the Fiasco.OC-specific assert.h.

Issue #1993

repos/base/src/lib/base/rpc_entrypoint.cc

@@ -19,6 +19,7 @@
 
 /* base-internal includes */
 #include <base/internal/ipc_server.h>
+#include <signal_source/signal_source.h>
 
 using namespace Genode;
 

repos/base/src/lib/base/signal.cc

@@ -18,11 +18,11 @@
 #include <base/thread.h>
 #include <base/sleep.h>
 #include <base/trace/events.h>
-#include <signal_source/client.h>
 #include <util/volatile_object.h>
 
 /* base-internal includes */
 #include <base/internal/globals.h>
+#include <signal_source/client.h>
 
 using namespace Genode;