common: Include what we use in the header

Specifically, include syslog.h to grab the LOG_* macros.

Change-Id: I596549a43e3a714a115f55df405178fe5ef53f09
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/common.h b/common.h
index 3e22dd2..0bd99de 100644
--- a/common.h
+++ b/common.h
@@ -6,6 +6,7 @@
 #include <stdarg.h>
 #include <stdbool.h>
 #include <stdint.h>
+#include <syslog.h>
 
 #ifndef PREFIX
 #define PREFIX ""