package/redis-plus-plus: needs threads

redis-plus-plus unconditonally uses mutex and so needs threads since its
addition in commit c46df9de21e718a9ed6a2df2bb945c386695d3bf and
https://github.com/sewenew/redis-plus-plus/commit/1fa8f6bbfb24c8f328aa23ee1dd3c3f99c1b4caf:

In file included from /home/autobuild/autobuild/instance-5/output-1/build/redis-plus-plus-1.3.10/src/sw/redis++/connection_pool.h:27,
                 from /home/autobuild/autobuild/instance-5/output-1/build/redis-plus-plus-1.3.10/src/sw/redis++/connection_pool.cpp:17:
/home/autobuild/autobuild/instance-5/output-1/build/redis-plus-plus-1.3.10/src/sw/redis++/sentinel.h:95:10: error: 'mutex' in namespace 'std' does not name a type
   95 |     std::mutex _mutex;
      |          ^~~~~

Fixes: c46df9de21e718a9ed6a2df2bb945c386695d3bf
 - http://autobuild.buildroot.org/results/cc4c54134e8a522b4c028aab4266f47cf862e2a9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 044752f5631a6fdb330ad57b8c8a5827c6b2bc86)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/redis-plus-plus/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_REDIS_PLUS_PLUS
 	bool "redis-plus-plus"
 	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	select BR2_PACKAGE_HIREDIS
 	help
 	  Redis client written in C++
@@ -11,5 +12,5 @@ config BR2_PACKAGE_REDIS_PLUS_PLUS
 
 	  https://github.com/sewenew/redis-plus-plus
 
-comment "redis-plus-plus needs a toolchain w/ C++"
-	depends on !BR2_INSTALL_LIBSTDCPP
+comment "redis-plus-plus needs a toolchain w/ C++, threads"
+	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS