Update grpc_ecosystem_grpc_gateway dependency

With the recent fixes, it makes this repository compatible with Bazel
flag `--incompatible_disallow_old_style_args_add`

Progress towards google#2

Closes google#5
googlecloudrobotics/core#5

ORIGINAL_AUTHOR=Laurent Le Brun <laurentlb@gmail.com>
GitOrigin-RevId: 0e362c40d00ceada2e301ad620c336c6346337f1
Change-Id: I96a7a45b4383d053e9177f05ad5870646a83827d

WORKSPACE

@@ -412,7 +412,7 @@ go_repository(
 
 go_repository(
     name = "grpc_ecosystem_grpc_gateway",
-    commit = "d8ad87ee91e1062c2a6117bda0b5523e9cb949ef",
+    commit = "9a7c952d3c7e6c0239ddb5d1d351e0fdb6c3c8d3",
     importpath = "github.com/grpc-ecosystem/grpc-gateway",
 )