Linux: fix build with system libevent.
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: de016e52bd170d2d4f2344f9bf92d50478b649e0@98496 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,adobe/chromium,yitian134/chromium