Fix the Fbcode suffixed targets

Summary:
D66676179 broke the build for `xplat/quic/api:*Fbcode`.

We switched from `raw_headers` to `exported_headers`, so fix the `header_namespace`.

Differential Revision: D67056078

fbshipit-source-id: db826c811186eea1a0b90a8ff330d4c4ba313c51

quic/defs.bzl

@@ -117,6 +117,7 @@ def mvfst_cpp_library(
             exported_deps = exported_deps,
             exported_headers = headers,
             headers = private_headers,
+            header_namespace = _compute_header_namespace(),
             visibility = ["PUBLIC"],
             **kwargs
         )