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/.gitignore b/.gitignore
index df85917..167d1e2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -42,6 +42,8 @@
 Doxyfile.codec
 Doxyfile.coding
 Doxyfile.gb
+Doxyfile.sim
+Doxyfile.usb
 
 debian/autoreconf.after
 debian/autoreconf.before
@@ -83,6 +85,8 @@
 doc/gsm
 doc/isdn
 doc/gb
+doc/sim
+doc/usb
 doc/html.tar
 doc/*.tag
 doc/*.tag.prep