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.es


Signed-off-by: Adeodato Simó <dato@net.com.org.es>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>