Plumbed copts (including the crucial -std=c99) to upb_proto_library() aspect.

BUILD

diff --git a/BUILD b/BUILD
index dccd531..6c9e9a1 100644
--- a/BUILD
+++ b/BUILD
@@ -8,6 +8,7 @@
     "upb_fasttable_enabled",
     "upb_proto_library",
     "upb_proto_reflection_library",
+    "upb_proto_library_copts",
 )
 
 # copybara:strip_for_google3_begin
@@ -36,6 +37,12 @@
     visibility = ["//visibility:public"],
 )
 
+upb_proto_library_copts(
+    name = "upb_proto_library_copts__for_generated_code_only_do_not_use",
+    copts = UPB_DEFAULT_COPTS,
+    visibility = ["//visibility:public"],
+)
+
 # Public C/C++ libraries #######################################################
 
 cc_library(