Fix for some rules not being run if their trigger is also an action dependency

The issue happens with e.g. accessibility.gyp for the ax_gen target:
Because ax_enums.idl is also an action additional input, it gets added to the
excluded_sources list when gyp generates the project, so the genapi_idl rule
on ax_enums.idl is marked as excluded from the build.

Rule target files should be removed from the excluded_sources list to
allow their rule to be run.

BUG=

R=scottmg@chromium.org

Patch from romain.pokrzywka@gmail.com.

Review URL: https://codereview.chromium.org/298703008

git-svn-id: http://gyp.googlecode.com/svn/trunk@1923 78cadc50-ecff-11dd-a971-7dbc132099af