boost: locale needs thread with icu

output/host/usr/bin/nios2-linux-readelf -d output/staging/usr/lib/libboost_locale.so

Dynamic section at offset 0x125ec0 contains 36 entries:
  Tag        Type                         Name/Value
 0x00000001 (NEEDED)                     Shared library: [libboost_chrono.so.1.67.0]
 0x00000001 (NEEDED)                     Shared library: [libboost_thread.so.1.67.0]
 0x00000001 (NEEDED)                     Shared library: [libboost_system.so.1.67.0]
 0x00000001 (NEEDED)                     Shared library: [librt.so.1]
 0x00000001 (NEEDED)                     Shared library: [libicudata.so.60]
 0x00000001 (NEEDED)                     Shared library: [libicui18n.so.60]
 0x00000001 (NEEDED)                     Shared library: [libicuuc.so.60]

Fixes:
 - http://autobuild.buildroot.net/results/57838f7cd84f37b66ab7007deaea847af8f54b72

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

package/boost/Config.in

@@ -190,6 +190,7 @@ config BR2_PACKAGE_BOOST_LOCALE
 	# details.
 	depends on !(BR2_STATIC_LIBS && BR2_PACKAGE_ICU)
 	select BR2_PACKAGE_BOOST_SYSTEM
+	select BR2_PACKAGE_BOOST_THREAD if BR2_PACKAGE_ICU
 	select BR2_PACKAGE_LIBICONV if !BR2_ENABLE_LOCALE
 	help
 	  Provide localization and Unicode handling tools for C++.