lcms2: Don't hard code Win32 platform on Windows

Change-Id: I60701b2c0a0ec06ada24b397107b337676dbd319
Reviewed-on: https://gerrit.libreoffice.org/13229
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>