[util/net] Fix append to undefined `defines`
[util/net] Fix append to undefined `defines`

From the Fuchsia build:
```
 ERROR at //third_party/crashpad/util/BUILD.gn:658:7: Undefined identifier.
       defines += [ "CRASHPAD_USE_BORINGSSL" ]
       ^------
 See //src/developer/forensics/crash_reports/BUILD.gn:127:5: which caused the file to be included.
     "//third_party/crashpad/util",
     ^----------------------------
```

Change-Id: I1f563e5bb599b3a7a83ee8211037ee1d7464bd62
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3370891
Reviewed-by: Francois Rousseau <abdb75be6d92482eb3ea75e9341306a6fcc3aacd@google.com>
Commit-Queue: Francois Rousseau <abdb75be6d92482eb3ea75e9341306a6fcc3aacd@google.com>
Reviewed-by: Robert Sesek <f55ad94e336f3a28322d0a25d280221ea9daf7a3@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad