Fix use of DESTDIR with PREFIX in meson

[ThmsP]

Proposed Changes

I had to package SU2 by rpm. Doing so I used DESTDIR and PREFIX :

./meson.py build --prefix=%{_prefix}
DESTDIR=%{buildroot} ./ninja -C build install

Everything works fine except I had to do a small fix in this file :SU2_PY/pySU2/install.sh

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