runtime: fix netbsd again after reorg

This time for sure.
That C file sure looked like a header file to me. :-)

R=golang-dev
CC=golang-dev
https://golang.org/cl/7830043

src/pkg/runtime/os_netbsd.c

@@ -30,7 +30,6 @@ extern void runtime·lwp_mcontext_init(void *mc, void *stack, M *mp, G *gp, void
 extern int32 runtime·lwp_park(Timespec *abstime, int32 unpark, void *hint, void *unparkhint);
 extern int32 runtime·lwp_unpark(int32 lwp, void *hint);
 extern int32 runtime·lwp_self(void);
-extern void runtime·lwp_tramp(void);
 
 // From NetBSD's <sys/sysctl.h>
 #define	CTL_HW	6

src/pkg/runtime/os_netbsd.h

@@ -17,6 +17,7 @@ void	runtime·sigaction(int32, struct sigaction*, struct sigaction*);
 void	runtime·sigaltstack(Sigaltstack*, Sigaltstack*);
 void	runtime·sigprocmask(int32, Sigset*, Sigset*);
 int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
+extern void runtime·lwp_tramp(void);
 
 #define	NSIG 33
 #define	SI_USER	0