Fixing build problems due to endian.h

Have to define _DEFAULT_SOURCE before you can use the endian
conversion functions.  As most standard library headers use
features.h, and _DEFAULT_SOURCE must be defined before features.h is
included, we have to include base.h before other headers.

lib/base.c

@@ -10,11 +10,10 @@
  * Description: Implementation of basic library functions
  */
 
-#include <endian.h>
-#include <string.h>
-
 #include "./base.h"
 
+#include <string.h>
+
 hword convert_bytes_to_hword(byte *bytes)
 {
   hword be_h = 0;