net/9p: include trans_common.h to fix missing prototype warning.

This silences -Wmissing-prototypes when defining p9_release_pages.

Link: http://lkml.kernel.org/r/b1c4df8f21689b10d451c28fe38e860722d20e71.1542089696.git.dato@net.com.org.esSigned-off-by: Adeodato Simó <dato@net.com.org.es>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>

net/9p/trans_common.c

@@ -14,6 +14,7 @@
 
 #include <linux/mm.h>
 #include <linux/module.h>
+#include "trans_common.h"
 
 /**
  *  p9_release_pages - Release pages after the transaction.