slang: Disable parallel make install

slang: Disable parallel make install

Installation task fails if run in parallel. This case happens if we
define PARALLEL_MAKEINST to a different value of PARALLEL_MAKE.

(From OE-Core rev: bbe59d7c49b540d65c871666c95cc89a23cab474)

(From OE-Core rev: 1aff7c867c0ad926f204459b65d3088900871ec9)

Signed-off-by: David Vincent <freesilicon@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>