Rename target_name to all_extensions.
Rename target_name to all_extensions.

There is target all in src/build/all.gyp, which will cause ninja
warning: multiple rules generate all.

crosswalk-project/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk