filter: make exit status local

It's only used in one place, and not useful to have around since
close_filter will die() if exit_status isn't what it expects, anyway. So
this is best as just a local variable instead of as part of the struct.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgit.h

@@ -62,7 +62,6 @@ struct cgit_filter {
 	int old_stdout;
 	int pipe_fh[2];
 	int pid;
-	int exitstatus;
 };
 
 struct cgit_repo {