reftests: Fix building on Visual Studio

Hi,

From the commit message:

We need to mark the prototypes in the headers so that they get exported properly from the reftest DLLs that we build, so that the reftest DLLs and program will link properly. Include gtk-reftest.h in reftest-snapshot.c so that the compiler picks up the export directives as needed.

I have chosen G_MODULE_EXPORT as it is the macro that is used in testsuite/reftests to export symbols as needed.

With blessings, thank you!