Make.pkg: use installed runtime.h for cgo

Fixes #1381.

R=r, r2
CC=golang-dev
https://golang.org/cl/3775050

src/Make.pkg

@@ -157,7 +157,7 @@ _CGO_LDFLAGS_darwin=-dynamiclib -Wl,-undefined,dynamic_lookup
 _CGO_LDFLAGS_windows=-shared -lm -mthreads
 
 # Have to compile the runtime header.
-RUNTIME_CFLAGS=-I"$(GOROOT)/src/pkg/runtime"
+RUNTIME_CFLAGS=-I"$(pkgdir)"
 
 # Compile _cgo_defun.c with 6c; needs access to the runtime headers.
 _cgo_defun.$O: _cgo_defun.c