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/debian/libosmousb-doc.install b/debian/libosmousb-doc.install
new file mode 100644
index 0000000..081159a
--- /dev/null
+++ b/debian/libosmousb-doc.install
@@ -0,0 +1 @@
+usr/share/doc/libosmocore/usb/