Enable -Werror for qtdeclarative

This allows CI to screen for warnings in src and tools code. It does
not apply to examples or tests.

This is the same as commit 7012db159bc4196444ace26991b24a31e3fba495 in
qtbase.

Change-Id: I0ea00fbd4b45156dedabfd16a3cb3e5f6170f23f
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>