Allow "make test" to run with Bourne shell.

protoc-gen-go/testdata/Makefile

diff --git a/protoc-gen-go/testdata/Makefile b/protoc-gen-go/testdata/Makefile
index 105cdec..b1ac45c 100644
--- a/protoc-gen-go/testdata/Makefile
+++ b/protoc-gen-go/testdata/Makefile
@@ -58,7 +58,7 @@
 
 regenerate:
 	# Invoke protoc once to generate three independent .pb.go files in the same package.
-	protoc --go_out=. multi/multi{1,2,3}.proto
+	protoc --go_out=. multi/multi1.proto multi/multi2.proto multi/multi3.proto
 
 #extension_test:	extension_test.$O
 #	$(LD) -L. -o $@ $<