Import proto_library.gni when rtc_enable_protobuf is true

Import proto_library.gni when rtc_enable_protobuf is true instead of when
build_with_mozilla is false.

Makes it maybe easier to reason about the intention (e.g. intention is to not
compile any protobuf in, hence flag rtc_enable_protobuf)

The build file could not work if build_with_mozilla = true but
rtc_enable_protobuf = true.

Bug: webrtc:10338
Change-Id: I26e5983bd1519aa46c308b11796d518de5ef7597
Reviewed-on: https://webrtc-review.googlesource.com/c/123763
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26868}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index 3755967..f909650 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -5,9 +5,10 @@
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
-
-import("//third_party/protobuf/proto_library.gni")
 import("../webrtc.gni")
+if (rtc_enable_protobuf) {
+  import("//third_party/protobuf/proto_library.gni")
+}
 
 group("rtc_tools") {
   # This target shall build all targets in tools/.