add GCC as dependency for NESTML

packages/py-nestml/package.py

@@ -34,3 +34,4 @@ class PyNestml(PythonPackage):
     depends_on('py-odetoolbox', type=('build', 'run'))
     depends_on('nest', type=('build', 'run'))
     depends_on('py-pytest', type='test')
+    depends_on('gcc', type=('build', 'test', 'run'))    # this should be whatever compiler is used for the env