Doxygen: exclude osmocom/core/gsm/protocol/*

We don't document the header files that define structs/enums/types
that are taken 1:1 from the relevant GSM specifications.  So let's avoid
having doxygen generate tons of warnings for related undocumented
members.

Change-Id: I231f78972a015902e4085aa80fcbc04009ae9f13
1 file changed