Squelch warnings from gyp where certain exclusion directives are used

When AdditionalDependencies! and DelayLoadDLLs! directives are used e.g.
in chrome_dll.gypi, warnings are emitted.  This CL supresses them.

BUG=246570
TEST=build\gyp_chromium with GYP_GENERATORS=msvs, no warnings generated
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/16690005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1650 78cadc50-ecff-11dd-a971-7dbc132099af