Fix `defines` in testing/fuzzers/BUILD.gn

Define `defines` for non-component builds so that we can add
`BUILD_WITH_CHROMIUM` to both component and non-component builds
later.

Bug: chromium:1433189
Change-Id: I5f42637ea554e2f554c3ae819664eef64a1740a7
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/106490
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nigi <nigi@chromium.org>

testing/fuzzers/BUILD.gn

diff --git a/testing/fuzzers/BUILD.gn b/testing/fuzzers/BUILD.gn
index 93cdf06..35624cb 100644
--- a/testing/fuzzers/BUILD.gn
+++ b/testing/fuzzers/BUILD.gn
@@ -222,6 +222,7 @@
   source_set(impl_name) {
     testonly = true
     sources = invoker.sources
+    defines = []
     deps = []
     if (defined(invoker.deps)) {
       deps += invoker.deps
@@ -234,7 +235,7 @@
       } else {
         export = "__attribute__((visibility(\"default\")))"
       }
-      defines = [ "LLVMFuzzerTestOneInput=${export} ${template_target_name}" ]
+      defines += [ "LLVMFuzzerTestOneInput=${export} ${template_target_name}" ]
       deps += [ "../../:pdfium_public_headers" ]
     } else {
       testonly = true