Linux: fix build with system libevent.

The libevent gyp target is used by both #host and #target
gyp targets, which are present even when not cross-compiling.
Mark the system libevent as providing both.

This is upstreaming a Gentoo Linux patch.

BUG=none

Review URL: http://codereview.chromium.org/7763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@98496 0039d316-1c4b-4281-b951-d872f2087c98

third_party/libevent/libevent.gyp

@@ -66,6 +66,7 @@
         {
           'target_name': 'libevent',
           'type': 'settings',
+          'toolsets': ['host', 'target'],
           'direct_dependent_settings': {
             'defines': [
               'USE_SYSTEM_LIBEVENT',