util: add u_atomic_test to .gitignore
util: add u_atomic_test to .gitignore

Signed-off-by: Juha-Pekka Heikkila <cab67866509becb852d2a4e6f7e5924ad04fb975@gmail.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler