Fix all warnings and enable -Werror for gptfdisk.

Bug: 26936282
Change-Id: Id7e9ec5e769e1abedc1d2e36bff2c5488b1c4b3e
diff --git a/gptpart.cc b/gptpart.cc
index 17d6f15..7e89909 100644
--- a/gptpart.cc
+++ b/gptpart.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
 
 #ifdef USE_UTF16
 #include <unicode/ustdio.h>