checkpath: Exclude csn1 files

CSN.1 parsing code is inherited/co-maintained with wireshark.git code,
and used to have its own coding style, so avoid enforcing osmocom code
style in there.

Change-Id: Ib6e7d2dcc3d8f262092f0100e55cad397aa226f4

.checkpatch.conf

@@ -1 +1,4 @@
+--exclude ^src/csn1.c$
+--exclude ^src/csn1.h$
+--exclude ^src/csn1_(enc|dec).c$
 --exclude ^src/gsm_rlcmac.c$