Qmake: Rename emitBuildDirectoryChanged()

The method is used to reset the build directory if shadow building
becomes unavailable due to the Qt version changing. It does not
explicitly emit anything anymore.

Change-Id: I27b46b77f334be62f4db5f269529f8a2d62d834b
Reviewed-by: Daniel Teske <daniel.teske@digia.com>