webkitgtk: add gperf-native to DEPENDS to fix build

|   Could NOT find Gperf (missing: GPERF_EXECUTABLE) (Required is at least
|   version "3.0.1")

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>