isisd: change ISIS_METHOD to use C preprocessor

this fixes warnings from vtysh extract.pl by making sure the isis method
files always 'work'. (previously, extract.pl would grab unselected isis
method sources and then complain about missing headers)
diff --git a/isisd/isis_bpf.c b/isisd/isis_bpf.c
index 8c3602d..05f1138 100644
--- a/isisd/isis_bpf.c
+++ b/isisd/isis_bpf.c
@@ -21,6 +21,7 @@
  */
 
 #include <zebra.h>
+#if ISIS_METHOD == ISIS_METHOD_BPF
 #include <net/if.h>
 #include <netinet/if_ether.h>
 #include <sys/time.h>
@@ -339,3 +340,5 @@
 {
   return ISIS_OK;
 }
+
+#endif /* ISIS_METHOD == ISIS_METHOD_BPF */