Define BORINGSSL_IMPLEMENTATION in the Bazel build

One less patch Envoy needs to apply. It should only matter when building
as a shared library on Windows, but the CMake build sets it
unconditionally, so match.

Change-Id: I96a2696d587839048184c448c2324cab836138a4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/56726
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
diff --git a/util/BUILD.toplevel b/util/BUILD.toplevel
index a855aa5..2067864 100644
--- a/util/BUILD.toplevel
+++ b/util/BUILD.toplevel
@@ -97,7 +97,9 @@
     "-Wmissing-declarations",
 ]
 
-boringssl_copts = select({
+boringssl_copts = [
+    "-DBORINGSSL_IMPLEMENTATION",
+] + select({
     # We assume that non-Windows builds use a GCC-compatible toolchain and that
     # Windows builds do not.
     #