Fix compatibility of build_theme.sh with Dash
Summary:
The script build_sh didn't work with Dash necause of "[[". When running it, I got the following:
$ ./build_theme.sh
Installing into /home/gepardo/.local/share/themes/Breeze
./build_theme.sh: 111: [[: not found
cp: cannot create directory '': No such file or directory
Test Plan: After changing "[[" to "[", it worked fine and installed the theme
Reviewers: cblack, svuorela
Reviewed By: cblack, svuorela
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24447