Make.inc: fix build for tiny.

Suggested by paulzhol@gmail.com

R=rsc
CC=golang-dev
https://golang.org/cl/2122049

src/Make.inc

@@ -22,9 +22,10 @@ ifeq ($(GOOS),darwin)
 else ifeq ($(GOOS),freebsd)
 else ifeq ($(GOOS),linux)
 else ifeq ($(GOOS),nacl)
+else ifeq ($(GOOS),tiny)
 else ifeq ($(GOOS),windows)
 else
-$(error Invalid $$GOOS '$(GOOS)'; must be darwin, freebsd, linux, nacl, or windows)
+$(error Invalid $$GOOS '$(GOOS)'; must be darwin, freebsd, linux, nacl, tiny, or windows)
 endif
 
 ifeq ($(GOARCH),)