bazel: assert if '-Swift.h' files are not exactly 1 (#374)

Addressing envoyproxy/envoy-mobile#372 (comment)

Signed-off-by: Michael Rebello <me@michaelrebello.com>
Signed-off-by: JP Simard <jp@jpsim.com>

mobile/bazel/swift_static_framework.bzl

@@ -52,10 +52,17 @@ def _swift_static_framework_impl(ctx):
 
         # We can potentially simplify this if this change lands upstream:
         # https://github.com/bazelbuild/rules_swift/issues/291
-        objc_headers = archive[apple_common.Objc].header.to_list()
-        objc_header = objc_headers[-1]
-        if objc_header:
-            zip_args.append(_zip_header_arg(module_name, objc_header))
+        objc_headers = [
+            header
+            for header in archive[apple_common.Objc].header.to_list()
+            if header.path.endswith("-Swift.h")
+        ]
+
+        if len(objc_headers) == 1:
+            zip_args.append(_zip_header_arg(module_name, objc_headers[0]))
+        else:
+            header_names = [header.basename for header in objc_headers]
+            fail("Expected exactly 1 '-Swift.h' header, got {}".format(", ".join(header_names)))
 
         swiftdoc = swift_info.direct_swiftdocs[0]
         swiftmodule = swift_info.direct_swiftmodules[0]