pipe-filter-ii, pipe-filter-gi: Fix warnings on native Windows

[Bruno Haible]

This patch fixes clang warnings on native Windows:
  warning: implicit declaration of function '_endthreadex' is invalid in C99
  warning: implicit declaration of function '_beginthreadex' is invalid in C99


2020-09-06  Bruno Haible  <bruno@clisp.org>

        pipe-filter-ii, pipe-filter-gi: Fix warnings on native Windows.
        * lib/pipe-filter-ii.c: Include <process.h>.
        * lib/pipe-filter-gi.c: Likewise.

diff --git a/lib/pipe-filter-gi.c b/lib/pipe-filter-gi.c
index 8b6870f..f7f8f6e 100644
--- a/lib/pipe-filter-gi.c
+++ b/lib/pipe-filter-gi.c
@@ -28,6 +28,7 @@
 #include <unistd.h>
 #if defined _WIN32 && ! defined __CYGWIN__
 # include <windows.h>
+# include <process.h> /* _beginthreadex, _endthreadex */
 #else
 # include <signal.h>
 # include <sys/select.h>
diff --git a/lib/pipe-filter-ii.c b/lib/pipe-filter-ii.c
index 77a29dc..b29f802 100644
--- a/lib/pipe-filter-ii.c
+++ b/lib/pipe-filter-ii.c
@@ -27,6 +27,7 @@
 #include <unistd.h>
 #if defined _WIN32 && ! defined __CYGWIN__
 # include <windows.h>
+# include <process.h> /* _beginthreadex, _endthreadex */
 #elif defined __KLIBC__
 # define INCL_DOS
 # include <os2.h>