Do not define _ISOC11_SOURCE in parser-sming.y

It overrides _DEFAULT_SOURCE, thereby hiding the declaration of timegm
in <time.h>.  This will result in build failures with future compilers
that do not support implicit function declarations.

Submitted upstream:

<https://lists.ibr.cs.tu-bs.de/hyperkitty/list/libsmi@ibr.cs.tu-bs.de/thread/MI6XPAR7JE2AY6UZIMXA3Q7JGD635JRR/>

diff --git a/lib/parser-sming.y b/lib/parser-sming.y
index 555e570613ad689f..32bddeac93eac7e8 100644
--- a/lib/parser-sming.y
+++ b/lib/parser-sming.y
@@ -17,7 +17,7 @@
     
 #ifdef BACKEND_SMING
 
-#define _ISOC99_SOURCE
+
 #include <stdio.h>
 #include <errno.h>
 #include <stdlib.h>
diff -up a/lib/parser-yang.y b/lib/parser-yang.y
--- a/lib/parser-yang.y	2024-01-05 09:28:40.994847831 -0500
+++ b/lib/parser-yang.y	2024-01-05 09:28:54.247033152 -0500
@@ -20,7 +20,6 @@
     
 #ifdef BACKEND_YANG
 
-#define _ISOC99_SOURCE
 #include <stdio.h>
 #include <errno.h>
 #include <stdlib.h>