Accept empty struct member decls

struct S { /*nothing*/; int a; };

is an acceptable struct declaration, there may be stray semicolons
in the member list.

tests/tcctest.c

@@ -642,6 +642,11 @@ union union2 {
 
 struct struct1 st1, st2;
 
+struct empty_mem {
+    /* nothing */ ;
+    int x;
+};
+
 int main(int argc, char **argv)
 {
     string_test();