testsuite/gtk/builder.c: Fix running on Visual Studio

Merged Chun-wei Fan requested to merge fix.builder.msvc into master


From the commit message:

Explicitly say in the code that we are exporting the symbols so that when the test program runs, it can find the required builder symbols.

This is because the builder test program expects the symbols that it uses are exported.

With blessings, thank you!

Edited by Chun-wei Fan

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