Adjusting phoenix.pro to accept shadow build

When trying to install the application after a shadow building (i.e., *mkdir build; cd build; qmake ..*), we got errors while installing image resources (*install: cannot stat ‘/resources/images/fritzing_icon.png’: No such file or directory*) and translations (*find: 'translations': No such file or directory'*).

This change is intended to fix it.

Reference: http://qt-project.org/wiki/QMake-top-level-srcdir-and-builddir