libyang BUGFIX set feature macro for strdup() on OSX

Since OS X requires different value (600) of _XOPEN_SOURCE feature test macro for
strdup() than linux (500), better use _POSIX_C_SOURCE 200809L. It is available
since glibc 2.12 (released 2010) so it shouldn't be an issue.
diff --git a/tests/utests/test_hash_table.c b/tests/utests/test_hash_table.c
index 57e0de0..2f0265f 100644
--- a/tests/utests/test_hash_table.c
+++ b/tests/utests/test_hash_table.c
@@ -13,6 +13,7 @@
  */
 
 #define _GNU_SOURCE
+#define _POSIX_C_SOURCE 200809L /* strdup */
 
 #include <stdarg.h>
 #include <stddef.h>