py-gpaw: Fixed broken fftw linking in newer versions of py-gpaw (#12795)

var/spack/repos/builtin/packages/py-gpaw/package.py

@@ -99,3 +99,5 @@ def patch(self):
             if '+scalapack' in spec:
                 f.write("scalapack = True\n")
                 f.write("define_macros += {0}\n".format(scalapack_macros))
+            if '+fftw' in spec:
+                f.write("fftw = True\n")