Move util/flags.h into the testing target.

Change-Id: Ic3cf9db8ba2721fc2fc3addbf5f39386590dd93f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/46817
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/BUILD b/BUILD
index 3402447..480646f 100644
--- a/BUILD
+++ b/BUILD
@@ -59,7 +59,6 @@
         "re2/unicode_groups.cc",
         "re2/unicode_groups.h",
         "re2/walker-inl.h",
-        "util/flags.h",
         "util/logging.h",
         "util/mix.h",
         "util/mutex.h",
@@ -111,6 +110,7 @@
         "re2/testing/string_generator.h",
         "re2/testing/tester.h",
         "util/benchmark.h",
+        "util/flags.h",
         "util/malloc_counter.h",
         "util/pcre.h",
         "util/test.h",