gallivm: initialize init_native_targets_once_flag correctly
gallivm: initialize init_native_targets_once_flag correctly

Signed-off-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler