include: Fixing config file include from common.h

The generated config file does not necessarily lands into the source
tree (e.g., if build is outside the source folder).

Change-Id: I83bfa3078db1a51204860f047f5a6a17c3f760fc
diff --git a/include/ndn-cpp/c/common.h b/include/ndn-cpp/c/common.h
index c1706d5..54a080c 100644
--- a/include/ndn-cpp/c/common.h
+++ b/include/ndn-cpp/c/common.h
@@ -7,7 +7,7 @@
 #ifndef NDN_COMMON_H
 #define NDN_COMMON_H
 
-#include "../ndn-cpp-config.h"
+#include <ndn-cpp/ndn-cpp-config.h>
 #include <stdint.h>
 // TODO: Is stddef.h portable?
 #include <stddef.h>