Fix all warnings and enable -Werror for gptfdisk.

Bug: 26936282
Change-Id: Id7e9ec5e769e1abedc1d2e36bff2c5488b1c4b3e
diff --git a/diskio-unix.cc b/diskio-unix.cc
index af71cdb..75fad2e 100644
--- a/diskio-unix.cc
+++ b/diskio-unix.cc
@@ -13,7 +13,9 @@
 // under the terms of the GNU GPL version 2, as detailed in the COPYING file.
 
 #define __STDC_LIMIT_MACROS
+#ifndef __STDC_CONSTANT_MACROS
 #define __STDC_CONSTANT_MACROS
+#endif
 
 #include <sys/ioctl.h>
 #include <string.h>