yocto-space-optimize.inc: move space optimize from recipe webkitgtk

yocto-space-optimize.inc: move space optimize from recipe webkitgtk

The yocto-space-optimize.inc is used to allow turning off the debug compiler
options for a small set of recipes to reduce build on disk footprint
and package/sstate sizes [1]. Move space optimize from recipe webkitgtk
to it.

Due to commit [2], update comments to remove `Unless DEBUG_BUILD is enabled'

[1] https://github.com/openembedded/openembedded-core/commit/a0483b962dfbba051de2c0b1acbe268579a81f22
[2] https://github.com/openembedded/openembedded-core/commit/9badf68d78d995f7d5d4cf27e045f029fc6d4044

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>