Don't run gjs tests on sparc64

gjs doesn't seem to work there at all (#827815).

Silence missing-dep-for-interpreter error on sparc64, where we install
the script but do not attempt to run it.

debian/control

@@ -17,7 +17,7 @@ Build-Depends:
  e2fslibs-dev,
  elfutils,
  fuse,
- gjs,
+ gjs [!sparc64],
  gobject-introspection,
  gtk-doc-tools,
  libarchive-dev,
@@ -138,7 +138,7 @@ Depends:
  cpio,
  fuse,
  gir1.2-ostree-1.0,
- gjs,
+ gjs [!sparc64],
  ostree,
  python,
  ${misc:Depends},