mono-docs: Honor DESTDIR when installing XML docs

The DESTDIR envvar was not being taken into account and the XML doc file
ended up in a different place than the DLL.

Fix T7802

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D8687

meson/meson_csharp_docs.sh

@@ -4,4 +4,4 @@
 # as meson's library() call can't track files other than the generated
 # efl_mono.dll
 
-cp $1 $2
+cp "$1" "$DESTDIR$2"