Fix missing parenthesis in a previous substitution '$(PREFIX_ROOT)' to  '$(CFG_MANDIR' on 1c4a348b072a
auto merge of #10193 : osaut/rust/Fix_install, r=huonw

Fix missing parentheses coming from a previous substitution '$(PREFIX_ROOT)' to  '$(CFG_MANDIR' on 1c4a348b072a