[MINOR] export the hostname variable so that all the code can access it

The hostname variable will be used later, export it.
diff --git a/include/common/defaults.h b/include/common/defaults.h
index a2267b1..f0873cf 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -179,4 +179,9 @@
 #define MAX_SIGNAL 256
 #endif
 
+/* Maximum host name length */
+#ifndef MAX_HOSTNAME_LEN
+#define MAX_HOSTNAME_LEN	32
+#endif
+
 #endif /* _COMMON_DEFAULTS_H */