webkitgtk: fix perl-native dependency

Currently, perl-native is missing from DEPENDS for webkitgtk even though
perlnative bbclass is inherited. This happens because the DEPENDS variable is
reassigned right after perlnative class is inherited:

inherit perlnative (DEPENDS += "perl-native")
...
DEPENDS = " \
            ..."

Adjust the DEPENDS line to use += in order to fix this.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>