patch 7.4.2106

Problem:    Clang warns about missing field in initializer.
Solution:   Define COMMA and use it.  (Kazunobu Kuriyama)

src/vim.h

@@ -1765,6 +1765,7 @@ int vim_memcmp(void *, void *, size_t);
 # ifndef INIT
 #  define INIT(x) x
 #  define DO_INIT
+#  define COMMA ,
 # endif
 #endif