install.sh: Install executable etc-files correctly

Commit 96c17c9 ("install.sh: Install files from etc/ implicitly")
missed the fact, that etc/mxq/gpu-policy is an executable script and
should be install with +x.

So check, whether the source file is executable and if so, then install
it as executable.

install.sh

@@ -104,7 +104,11 @@ function install_symlink()
 function install_etc_files()
 {
     (cd etc ; find * -type f) | while read -r path; do
-        install_data etc/$path "$DESTDIR$sysconfdir/$path"
+        if [[ -x etc/$path ]]; then
+            install_exec etc/$path "$DESTDIR$sysconfdir/$path"
+        else
+            install_data etc/$path "$DESTDIR$sysconfdir/$path"
+        fi
     done
 }