[PATCH] Fix non-Linux build

As of 509f5b0d, the build fails on Darwin/macOS, which does not have 
memfd_create(2). The build fails on redir.c because the fallback for 
memfd_create, which is defined in system.h, is not included. Presumably it is 
the same on other systems without memfd_create.

diff --git a/src/redir.c b/src/redir.c
index 2505d49..8a77a92 100644
--- a/src/redir.c
+++ b/src/redir.c
@@ -57,6 +57,7 @@
  #include "memalloc.h"
  #include "error.h"
  #include "trap.h"
+#include "system.h"


  #define EMPTY -2		/* marks an unused slot in redirtab */

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