ospfd: compile warning cleanups

A set of patches to clarify some comments as well as cleanup code that was
causing warnings.  After these patches, the code can be compiled with

-Wall -Wsign-compare -Wpointer-arith -Wbad-function-cast -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations -Wchar-subscripts -Wcast-qual
-Wextra -Wno-unused-parameter -Wno-missing-field-initializers

(what is current in trunk plus -Wextra -Wno-unused-parameter
-Wno-missing-field-initializers).

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
diff --git a/ospfd/ospf_lsa.h b/ospfd/ospf_lsa.h
index 6c95ff1..9ff2d92 100644
--- a/ospfd/ospf_lsa.h
+++ b/ospfd/ospf_lsa.h
@@ -243,6 +243,7 @@
 
 extern int get_age (struct ospf_lsa *);
 extern u_int16_t ospf_lsa_checksum (struct lsa_header *);
+extern int ospf_lsa_checksum_valid (struct lsa_header *);
 extern int ospf_lsa_refresh_delay (struct ospf_lsa *);
 
 extern const char *dump_lsa_key (struct ospf_lsa *);