doc: Add missing include in example code

It's not broken because the header file is included via other headers,
but for better style we should be more explicit.

Reported-by: mthrockmorton@hme.com

doc/src/sgml/trigger.sgml

@@ -705,6 +705,7 @@ CREATE TABLE ttest (
     This is the source code of the trigger function:
 <programlisting><![CDATA[
 #include "postgres.h"
+#include "fmgr.h"
 #include "executor/spi.h"       /* this is what you need to work with SPI */
 #include "commands/trigger.h"   /* ... triggers ... */
 #include "utils/rel.h"          /* ... and relations */