add flags for find libevent header files not in standard include path

diff --git a/ipc/chromium/moz.build b/ipc/chromium/moz.build
index d29817b..70252e6 100644
--- a/ipc/chromium/moz.build
+++ b/ipc/chromium/moz.build
@@ -118,6 +118,9 @@ if os_solaris:
 
 CXXFLAGS += CONFIG['TK_CFLAGS']
 
+if CONFIG['MOZ_SYSTEM_LIBEVENT']:
+    CXXFLAGS += CONFIG['MOZ_LIBEVENT_CFLAGS']
+
 TEST_DIRS += [
     "gtest",
 ]