Fix Chromium build - set is_freebsd variable (#3565)

#3467 broke Chromium builds
because Chromium intentionally doesn't define is_freebsd.

Example failure:
https://chromium-review.googlesource.com/c/chromium/src/+/7122439

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---------

Co-authored-by: Primiano Tucci <[email protected]>

Author: kraskevich

gn/perfetto.gni

@@ -78,6 +78,10 @@ if (!defined(is_qnx)) {
   is_qnx = false
 }
 
+if (!defined(is_freebsd)) {
+  is_freebsd = false
+}
+
 declare_args() {
   # The Android blueprint file generator set this to true (as well as
   # is_perfetto_build_generator). This is just about being built in the