package/rtl_433: fix build without threads

Fix the following build failure without threads raised since bump to
version 22.11 in commit 6f848c06:

In file included from /home/buildroot/autobuild/instance-3/output-1/build/rtl_433-22.11/src/data_tag.c:17:
/home/buildroot/autobuild/instance-3/output-1/build/rtl_433-22.11/include/mongoose.h:407:10: fatal error: pthread.h: No such file or directory
  407 | #include <pthread.h>
      |          ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/9f1677fc5a4568be0b9c121060c5d821ac2ae21b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>