boost: timer needs chrono and system

output/host/usr/bin/m68k-linux-readelf -d output/staging/usr/lib/libboost_timer.so

Dynamic section at offset 0x5ef8 contains 29 entries:
  Tag        Type                         Name/Value
 0x00000001 (NEEDED)                     Shared library: [libboost_chrono.so.1.67.0]
 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

@@ -300,6 +300,8 @@ config BR2_PACKAGE_BOOST_THREAD
 
 config BR2_PACKAGE_BOOST_TIMER
 	bool "boost-timer"
+	select BR2_PACKAGE_BOOST_CHRONO
+	select BR2_PACKAGE_BOOST_SYSTEM
 	help
 	  Event timer, progress timer, and progress display classes.