boost: contract needs system

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

Dynamic section at offset 0x19f00 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

@@ -90,6 +90,7 @@ config BR2_PACKAGE_BOOST_CONTRACT
 	bool "boost-contract"
 	# pthread_condattr_setclock
 	depends on BR2_TOOLCHAIN_HAS_THREADS_NPTL
+	select BR2_PACKAGE_BOOST_SYSTEM
 	help
 	  Contract programming for C++.