tst-difftime.c: Use "main (void)"

Use "main (void)" instead of "main (void)" to avoid GCC 4.9 warning:

tst-difftime.c:62:1: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
 main ()
 ^
tst-difftime.c: In function ‘main’:
tst-difftime.c:62:1: error: old-style function definition [-Werror=old-style-definition]
cc1: all warnings being treated as errors

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>

time/tst-difftime.c

@@ -59,7 +59,7 @@ do_test (void)
 #include <support/test-driver.h>
 
 int
-main ()
+main (void)
 {
   return EXIT_UNSUPPORTED;
 }