Silence compiler warning in sepgsql

<selinux/label.h> includes <stdbool.h>, which creates an incompatible
We don't care if <stdbool.h> redefines "true"/"false"; those are close
enough.

Complaint and initial patch by Mike Palmiotto. Final approach per
Tom Lane's suggestion, as discussed on hackers. Backpatching to
all supported branches.

Discussion: https://postgr.es/m/flat/623bcaae-112e-ced0-8c22-a84f75ae0c53%40joeconway.com

contrib/sepgsql/label.c

@@ -10,6 +10,16 @@
  */
 #include "postgres.h"
 
+#include <selinux/label.h>
+
+/*
+ * <selinux/label.h> includes <stdbool.h>, which creates an incompatible
+ * #define for bool.  Get rid of that so we can use our own typedef.
+ * (We don't care if <stdbool.h> redefines "true"/"false"; those are close
+ * enough.)
+ */
+#undef bool
+
 #include "access/heapam.h"
 #include "access/htup_details.h"
 #include "access/genam.h"
@@ -37,8 +47,6 @@
 
 #include "sepgsql.h"
 
-#include <selinux/label.h>
-
 /*
  * Saved hook entries (if stacked)
  */