boost: filesystem needs system

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

Dynamic section at offset 0x17f00 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

@@ -145,6 +145,7 @@ comment "boost-fiber needs a toolchain not affected by GCC bug 64735, 85180"
 
 config BR2_PACKAGE_BOOST_FILESYSTEM
 	bool "boost-filesystem"
+	select BR2_PACKAGE_BOOST_SYSTEM
 	help
 	  The Boost Filesystem Library provides portable facilities to
 	  query and manipulate paths, files, and directories.