egl/wayland: define set_cloexec_or_close only when mkostemp is not present

Fixes a compiler warning of defined but not used function when
HAVE_MKOSTEMP is defined.

Fixes: eb3e2562a4b(configure.ac: check for mkostemp())
Signed-off-by: Boyan Ding <boyan.j.ding@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>

src/egl/drivers/dri2/platform_wayland.c

@@ -1227,6 +1227,8 @@ dri2_wl_swrast_get_stride_for_format(int format, int w)
  * Taken from weston shared/os-compatibility.c
  */
 
+#ifndef HAVE_MKOSTEMP
+
 static int
 set_cloexec_or_close(int fd)
 {
@@ -1249,6 +1251,8 @@ err:
    return -1;
 }
 
+#endif
+
 /*
  * Taken from weston shared/os-compatibility.c
  */