Avoid a long obsolete rpm API use

dsFromHeader() and dsOfHeader() header methods were removed in rpm 4.19,
passing header to rpm.ds constructor is the preferred way since
rpm 4.8 / RHEL 6.

plugins/builddep.py

@@ -192,7 +192,7 @@ def _src_deps(self, src_fn):
             os.close(fd)
             raise dnf.exceptions.Error(e)
         os.close(fd)
-        ds = h.dsFromHeader('requirename')
+        ds = rpm.ds(h, 'requirename')
         done = True
         for dep in ds:
             reldep_str = self._rpm_dep2reldep_str(dep)