global.h: undefine utash_fatal before re-defining

Remvove compilation warning when run with 'UTASH' enabled.

Change-Id: I032613ec5704dde1674de20676144a5969a969a9
Signed-off-by: Gaëtan Harter <gaetan.harter@inria.fr>
diff --git a/global.h b/global.h
index c2ce904..dd3a77f 100644
--- a/global.h
+++ b/global.h
@@ -136,6 +136,7 @@
 }
 #endif /* HAVE_FLS */
 
+#undef uthash_fatal
 #define uthash_fatal(msg) return(-1) /* fatal error in uthash */
 
 #endif /* _DTLS_GLOBAL_H_ */