Add conflict statement to sda as suggested

As suggested by @emuller (sorry for the mention spam today...), I added a conflict statement.

packages/sda/package.py

@@ -29,6 +29,7 @@ class Sda(MakefilePackage):
     # Dependencies.
     depends_on('%gcc@9.4.0:')
     depends_on('python@3.8:', type=('build','run'))
+    conflicts('%gcc@:9.3.999', when='@7.3.3d')
     conflicts('%intel')
 
     build_directory = 'src'