doxygen: add missing gb API doc generation

Files in include/osmocom/gprs/ and src/gb/ are not included in any doxygen
generated API docs. Add Doxyfile.gb.in and adjust configure.ac and Makefile.am.

Change-Id: Ieb64f497f55368e396872083237c9ff28da2dd93
diff --git a/configure.ac b/configure.ac
index 340a896..1e258a4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -308,4 +308,5 @@
 	Doxyfile.vty
 	Doxyfile.codec
 	Doxyfile.coding
+	Doxyfile.gb
 	Makefile)