curl: fix disable threaded resolver

Bump to 7.55.1 broke the disable threaded resolver feature as reported
in https://github.com/curl/curl/issues/1784

.
As a result curl is always compiled with the threaded resolver feature
enabled which causes a dependency issue on pthread for uclibc.
Fix this issue by backporting the upstream curl commit which fixes
disable threaded resolver.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>