webkitgtk: Add upstream patch to ensure CMAKE_BUILD_TYPE is honored

Make WebKitGTK+ honor the value of CMAKE_BUILD_TYPE defined in the CMake
toolchain file by backporting the following upstream WebKit patch:

    https://trac.webkit.org/changeset/225168



This reduces the generated binary sizes when building in "Release" mode
(BR2_ENABLE_DEBUG=n), for example when targeting ARMv8 the size reduction
is ~17 MiB.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>