MINOR: log: make MAX_SYSLOG_LEN overridable at build time

This value was set in log.h without any #ifndef around, so when one
wanted to change it, a patch was needed. Let's move it to defaults.h
with the usual #ifndef so that it's easier to change it.
diff --git a/include/common/defaults.h b/include/common/defaults.h
index 5508314..6e5840e 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -48,6 +48,10 @@
 #define CAPTURE_LEN     64
 #endif
 
+#ifndef MAX_SYSLOG_LEN
+#define MAX_SYSLOG_LEN          1024
+#endif
+
 // maximum line size when parsing config
 #ifndef LINESIZE
 #define LINESIZE	2048