glslang: strip whitespace in pkgconfig file

Whilst pkg-config is fine with .pc files containing leading whitespace,
pkgconf is less forgiving.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 14bfe5f1)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>