boost: thread needs system

output/host/usr/bin/arm-buildroot-linux-uclibcgnueabihf-readelf -d output/staging/usr/lib/libboost_thread.so

Dynamic section at offset 0x21f00 contains 28 entries:
  Tag        Type                         Name/Value
 0x00000001 (NEEDED)                     Shared library: [libboost_system.so.1.67.0]

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/boost/Config.in

@@ -310,6 +310,7 @@ config BR2_PACKAGE_BOOST_TEST
 
 config BR2_PACKAGE_BOOST_THREAD
 	bool "boost-thread"
+	select BR2_PACKAGE_BOOST_SYSTEM
 	help
 	  Portable C++ multi-threading. C++11, C++14.