tzset: fix compilation warnings on mingw

[Bruno Haible]

This file has invocations of undeclared functions getenv() and strchr().
This patch fixes it.


2020-06-21  Bruno Haible  <bruno@clisp.org>

        tzset: Fix compilation warnings on mingw (regression from 2017-05-01).
        * lib/tzset.c: Include <stdlib.h>, <string.h>.

diff --git a/lib/tzset.c b/lib/tzset.c
index 24c972d..e3543ba 100644
--- a/lib/tzset.c
+++ b/lib/tzset.c
@@ -22,6 +22,9 @@
 /* Specification.  */
 #include <time.h>
 
+#include <stdlib.h>
+#include <string.h>
+
 #include "localtime-buffer.h"
 
 /* This is a wrapper for tzset, for systems on which tzset may clobber