[GnuTLS] Add gnutls-specific headers property. (#7259)

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

@@ -90,3 +90,9 @@ def configure_args(self):
             ])
 
         return args
+
+    @property
+    def headers(self):
+        headers = find_all_headers(self.prefix.include)
+        headers.directories = self.prefix.include
+        return headers