libjpeg/libjpeg.gyp should be used in both local and system libjpeg,
otherwise, header file won't found when build with system libjpeg. see
the comments in third_party/libjpeg/libjpeg.gyp

BUG=chromium:316499
R=michaelbai@chromium.org

Review URL: https://webrtc-codereview.appspot.com/5709004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@912 16f28f9a-4ce2-e073-06de-1de4eb20be90