sigrok-cross-mingw: Temporarily skip Pulseview PDF manual for Windows CI builds.

sigrok-cross-mingw: Temporarily skip Pulseview PDF manual for Windows CI builds.

Currently Windows CI jobs fail on sigrok.org build machines in the
Pulseview 'make manual-pdf' step. Intentionally break the lookup of the
external utility from the cross MinGW script, to skip the PDF creation.

This temporary evil hack needs to get reverted when the actual cause of
the failed build got addressed. The approach of the sigrok-util patch
keeps all other Pulseview builds unaffected.