doxygen: also generate documentation for libosmo{sim,usb}

We do have API documentation for all libraries, excepte these two.

Change-Id: I2ec5803bc219b9ceaf0d52d41a03ab78f580e689
diff --git a/configure.ac b/configure.ac
index ee1c6f0..ecff654 100644
--- a/configure.ac
+++ b/configure.ac
@@ -627,5 +627,7 @@
 	Doxyfile.coding
 	Doxyfile.gb
 	Doxyfile.ctrl
+	Doxyfile.sim
+	Doxyfile.usb
 	Makefile
 	contrib/libosmocore.spec)