change folly to use fbsource libevent alias

Summary:
have folly use an fbsource alias for libevent that will correctly redirect to the right 3p library.

#buildall

Reviewed By: 8Keep

Differential Revision: D116545919

fbshipit-source-id: 01de2fe976c3572de79010b3fcd418ea6d48e945

Author: DanielC314

folly/portability/BUCK

@@ -83,13 +83,11 @@ fb_dirsync_cpp_library(
     headers = ["Event.h"],
     use_raw_headers = True,
     exported_deps = [
+        "fbsource//third-party/libevent:libevent",
         ":fcntl",
         ":windows",
         "//folly/net/detail:socket_file_descriptor_map",
     ],
-    exported_external_deps = [
-        "libevent",
-    ],
 )
 
 fb_dirsync_cpp_library(