Add a generic abstraction for GSM/3G authentication algorithms

Indiidual algorithms can be implemented as plugins.  libosmogsm itself
only provides COMP128v1 via this generic interface.
diff --git a/include/osmocom/crypt/Makefile.am b/include/osmocom/crypt/Makefile.am
index 7ce69fd..e4a6e53 100644
--- a/include/osmocom/crypt/Makefile.am
+++ b/include/osmocom/crypt/Makefile.am
@@ -1,3 +1,3 @@
-osmocrypt_HEADERS = gprs_cipher.h
+osmocrypt_HEADERS = gprs_cipher.h auth.h
 
 osmocryptdir = $(includedir)/osmocom/crypt