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/.gitignore b/.gitignore
index 89459f0..f26b699 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,6 +38,7 @@
 Doxyfile.vty
 Doxyfile.codec
 Doxyfile.coding
+Doxyfile.gb
 
 debian/autoreconf.after
 debian/autoreconf.before
@@ -115,6 +116,7 @@
 doc/vty/html
 doc/vty/doxygen_sqlite3.db
 doc/gsm
+doc/gb
 doc/html.tar
 doc/*.tag
 tags