Fix missing dependency in ghostscript (#6866)

var/spack/repos/builtin/packages/ghostscript/package.py

@@ -43,6 +43,7 @@ class Ghostscript(AutotoolsPackage):
     depends_on('libpng')
     depends_on('libtiff')
     depends_on('zlib')
+    depends_on('libxext')
 
     def url_for_version(self, version):
         baseurl = "https://github.com/ArtifexSoftware/ghostpdl-downloads/releases/download/gs{0}/ghostscript-{1}.tar.gz"