Undefine QT_RESTRICTED_CAST_FROM_ASCII for headersclean build step

The module might add QT_RESTRICTED_CAST_FROM_ASCII to DEFINES to
be able to use string literals under the assumption they are UTF8
without warnings from QT_ASCII_CAST_WARN, but this conflicts with
QT_NO_CAST_FROM_ASCII which is added for the headersclean build
step.

Change-Id: Ic1d7b5415350477d751a6c15219d4f8feb816a3c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

mkspecs/features/qt_module_headers.prf

@@ -174,6 +174,7 @@ headersclean:!internal_module {
     # Make sure that the header compiles with our strict options
     hcleanDEFS = -DQT_NO_CAST_TO_ASCII=1 \
                  -DQT_NO_CAST_FROM_ASCII=1 \
+                 -UQT_RESTRICTED_CAST_FROM_ASCII \
                  -DQT_STRICT_ITERATORS \
                  -DQT_NO_URL_CAST_FROM_STRING=1 \
                  -DQT_NO_CAST_FROM_BYTEARRAY=1 \