Adding the #include to defs.c makes it get processed
by cmd/dist, which writes out Go equivalent for all the
C data structures defined in defs.c.

This in turn makes it necessary to define the Plink type,
used in os_plan9.h, in os_plan9.go. Rename it to _Plink
to avoid being exported.

LGTM=0intro, iant
R=golang-codereviews, iant, 0intro
CC=golang-codereviews, r
https://golang.org/cl/132490043