alwayslink=1 for reflection plugin

The reflection plugin uses a static initializer to
enable itself, but no one depends on its symbols, so it gets
optimized out.

Set alwayslink in the reflection plugin to fix that.
diff --git a/BUILD b/BUILD
index 26981ca..74d9a77 100644
--- a/BUILD
+++ b/BUILD
@@ -1522,6 +1522,7 @@
         ":grpc++",
         "//src/proto/grpc/reflection/v1alpha:reflection_proto",
     ],
+    alwayslink = 1,
 )
 
 grpc_cc_library(