xwalk-launcher: Simplify dependency on 'gio' gyp target.

Both Linux and Tizen depend on system.gyp:gio, so we can depend on it
unconditionally and simplify the gyp code.
Merge pull request #2665 from rakuco/simplify-gio-unix-gyp-dependency

xwalk-launcher: Simplify dependency on 'gio' gyp target.