package/libwebsockets: fix openssl build without threads

Fix the following openssl build failure without threads raised since
bump to version 4.3.1 in commit 3a9cf51a and
https://github.com/warmcat/libwebsockets/commit/471dbf5280b2d0770cdf2803642586cee37a335d:

In file included from /home/buildroot/autobuild/instance-0/output-1/build/libwebsockets-4.3.1/include/libwebsockets.h:661,
                 from /home/buildroot/autobuild/instance-0/output-1/build/libwebsockets-4.3.1/lib/core/./private-lib-core.h:140,
                 from /home/buildroot/autobuild/instance-0/output-1/build/libwebsockets-4.3.1/lib/plat/unix/unix-misc.c:28:
/home/buildroot/autobuild/instance-0/output-1/build/libwebsockets-4.3.1/include/libwebsockets/lws-genhash.h:85:18: error: field 'ctx' has incomplete type
   85 |         HMAC_CTX ctx;
      |                  ^~~

Fixes:
 - http://autobuild.buildroot.org/results/2ae9e3249b6fcc9e6c30e7783e264fc6599e61df



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>