jcphuff.c: Fix compiler warning with Visual C++

GCC doesn't seem to care if we pass no arguments to the ENCODE_COEFS*
macros, but MSVC does.  Insert a no-op to make it happy.
diff --git a/jcphuff.c b/jcphuff.c
index bcec852..b9a8de3 100644
--- a/jcphuff.c
+++ b/jcphuff.c
@@ -697,7 +697,7 @@
 
   /* Encode the AC coefficients per section G.1.2.2, fig. G.3 */
 
-  ENCODE_COEFS_AC_FIRST();
+  ENCODE_COEFS_AC_FIRST((void)0;);
 
 #if SIZEOF_SIZE_T == 4
   zerobits = bits[1];
@@ -960,7 +960,7 @@
 #else
   signbits = bits[2];
 #endif
-  ENCODE_COEFS_AC_REFINE();
+  ENCODE_COEFS_AC_REFINE((void)0;);
 
 #if SIZEOF_SIZE_T == 4
   zerobits = bits[1];