webkit-webos: Add gperf-native dependency

* gperf-native is usually built before webkit-webos, because it's
  dependency of eglibc, but in some cases (e.g. with external toolchain)
  it's not and webkit-webos fails as reported by Susan Montooth.

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: I1221ad030d8ef59b8513bf86cba5453263086d04
Reviewed-on: http://g2g.palm.com/1226
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Tested-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@palm.com>