test/misc/test-nftw.c: fix build with latest glibc

Fix the following error with latest glibc:

In function 'open',
    inlined from 'do_test' at tst-nftw.c:38:10,
    inlined from 'main' at ../test-skeleton.c:318:12:
/home/mark/buildroot-test/instance-0/output/host/riscv32-buildroot-linux-gnu/sysroot/usr/include/bits/fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
    __open_missing_mode ();
    ^~~~~~~~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.net/results/f370abcc8dc12975d96a46c34db978554f8c21db

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

test/misc/tst-nftw.c

@@ -35,9 +35,9 @@ do_test(void)
 	perror("Creating path");
     if ((mkdir(subpath, 0700)) < 0)
 	perror("Creating subpath");
-    if ((open(filepath, O_CREAT)) < 0)
+    if ((open(filepath, O_CREAT, 0600)) < 0)
 	perror("Opening filepath");
-    if ((open(filesubpath, O_CREAT)) < 0)
+    if ((open(filesubpath, O_CREAT, 0600)) < 0)
 	perror("Opening filesubpath");
 
     if (nftw(path, process_one_entry, 100, (FTW_CHDIR|FTW_DEPTH|FTW_PHYS)) < 0)