build: work with GNU Make 3.82

Fix building on GNU Make 3.82 (caused by mixed implicit and normal
rules).

The issue was introduced in changeset 6110:ca0beac3b543.

R=rsc
CC=golang-dev
https://golang.org/cl/2100044

src/Make.pkg

@@ -116,7 +116,10 @@ _cgo_defun.c: $(CGOFILES)
 	CGOPKGPATH=$(dir) cgo -- $(CGO_CFLAGS) $(CGOFILES)
 
 # Ugly but necessary
-_cgo_gotypes.go _cgo_export.c _cgo_export.h %.cgo1.go %.cgo2.c: _cgo_defun.c
+_cgo_gotypes.go _cgo_export.c _cgo_export.h: _cgo_defun.c
+	@true
+
+%.cgo1.go %.cgo2.c: _cgo_defun.c
 	@true
 
 %.cgo2.o: %.cgo2.c