Fix MS-Windows build broken by recent changes in lcms.c

Fix MS-Windows build broken by recent changes in lcms.c

* src/lcms.c [WINDOWSNT]: Define types for cmsWhitePointFromTemp
and cmsxyY2XYZ function pointers.
(init_lcms_functions) [WINDOWSNT]: Load cmsWhitePointFromTemp and
cmsxyY2XYZ from liblcms2.
(cmsWhitePointFromTemp, cmsxyY2XYZ) [WINDOWSNT]: Redirect to the
corresponding function pointers.
(Flcms_temp_to_white_point): Minor stylistic changes.  Doc fix.
(syms_of_lcms2): Defsubr Slcms_temp_to_white_point.