runtime: fix windows build

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/3419043
diff --git a/src/pkg/runtime/windows/os.h b/src/pkg/runtime/windows/os.h
index 445e5b5..3e071ea 100644
--- a/src/pkg/runtime/windows/os.h
+++ b/src/pkg/runtime/windows/os.h
@@ -4,6 +4,7 @@
 
 extern void *runtime·LoadLibraryEx;
 extern void *runtime·GetProcAddress;
+extern void *runtime·GetLastError;
 
 #define runtime·goargs runtime·windows_goargs
 void runtime·windows_goargs(void);