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

Doxyfile.gsm.in

@@ -654,7 +654,7 @@ EXCLUDE_SYMLINKS       = NO
 # against the file with absolute path, so to exclude all test directories
 # for example use the pattern */test/*
 
-EXCLUDE_PATTERNS       =
+EXCLUDE_PATTERNS       = */include/osmocom/gsm/protocol/*
 
 # The EXCLUDE_SYMBOLS tag can be used to specify one or more symbol names
 # (namespaces, classes, functions, etc.) that should be excluded from the