Move rtc_base/ignore_wundef.h to its own target.

Since rtc_base/ignore_wundef.h doesn't have any dependency, it is easy to
move it to its own target and allow its dependant to avoid to take a
dependency rtc_base:on rtc_base_approved.

Bug: webrtc:9419
Change-Id: I17f205b0cb2b21cad388b04e60082df9398dffdf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157428
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29548}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index b643344..f453ace 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -45,6 +45,12 @@
   ]
 }
 
+rtc_source_set("ignore_wundef") {
+  sources = [
+    "ignore_wundef.h",
+  ]
+}
+
 # The subset of rtc_base approved for use outside of libjingle.
 # TODO(bugs.webrtc.org/9838): Create small and focused build targets and remove
 # the old concept of rtc_base and rtc_base_approved.
@@ -79,7 +85,6 @@
     "copy_on_write_buffer.h",
     "event_tracer.cc",
     "event_tracer.h",
-    "ignore_wundef.h",
     "location.cc",
     "location.h",
     "message_buffer_reader.h",