[fuchsia] fix crashpad_use_boringssl_for_http_transport_socket path

* https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1745355/17/util/BUILD.gn#184 defines "defines" earlier in the target
* this only affects Fuchsia in the Fuchsia tree, not the Crashpad tree

TESTED=`fx build` in Fuchsia tree

Change-Id: I5ac454828f958d6de4e4c6788d7c9a31408dc732
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1796964
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Francois Rousseau <frousseau@google.com>
diff --git a/util/BUILD.gn b/util/BUILD.gn
index 853b15b..dec7d5c 100644
--- a/util/BUILD.gn
+++ b/util/BUILD.gn
@@ -279,7 +279,7 @@
   if (crashpad_is_linux || crashpad_is_fuchsia || crashpad_is_android) {
     sources += [ "net/http_transport_socket.cc" ]
     if (crashpad_use_boringssl_for_http_transport_socket) {
-      defines = [ "CRASHPAD_USE_BORINGSSL" ]
+      defines += [ "CRASHPAD_USE_BORINGSSL" ]
 
       if (crashpad_is_in_fuchsia) {
         deps += [ "//third_party/boringssl" ]