Remove redundant includes

These are all included in git-compat-util.h (when necessary), which we
include in cgit.h.

Signed-off-by: John Keeping <john@keeping.me.uk>

filter.c

@@ -8,13 +8,7 @@
 
 #include "cgit.h"
 #include "html.h"
-#include <sys/types.h>
-#include <sys/wait.h>
-#include <unistd.h>
-#include <string.h>
-#include <stdlib.h>
 #include <dlfcn.h>
-#include <errno.h>
 #ifndef NO_LUA
 #include <lua.h>
 #include <lualib.h>