A GLogLevelFlags fatal_mask variable was
created inside an #if defined (G_OS_WIN32)
block, below a function call to
gimp_test_utils_set_gimp3_directory (). This
resulted in build warnings on Windows:
"ISO C90 forbids mixing declarations and code"
This patch adds another #if defined (G_OS_WIN32)
at the top and moves the variable declaration there.