qmake5.bbclass: Add WEBOS_INSTALL_PREFIX

:Release Notes:
Add WEBOS_INSTALL_PREFIX to EXTRA_QMAKEVARS_PRE, the default parameters
for qmake.

:Detailed Notes:

:Testing Performed:
Verified in bitbake build that this variable passed to qmake and
Makefile generated based on this variable contained a correct path.

:QA Notes:

:Issues Addressed:
[GF-34262] webos_qmake5 bb-class should provide enough information about
           sysbus files installation paths
[GF-38915] CCC: starfish-luna-surface-manager=60 luna-surfacemanager=36
           qml-starfish-components=11 qml-webos-bridge=13
           qt-features-webos=10 qml-starfish-components=11

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Change-Id: I5a38cd3b3975c09e7c8a465c664a83108dda6f6f
Reviewed-on: https://g2g.palm.com/3089
Reviewed-by: Mikko Levonmaa <mikko.levonmaa@lge.com>
Tested-by: Mikko Levonmaa <mikko.levonmaa@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Jongho Kim <jongho1208.kim@lge.com>
Tested-by: Jongho Kim <jongho1208.kim@lge.com>