xz-utils: needs threads

It's one of the major features of the 5.2.0 release. Fixes:
http://autobuild.buildroot.net/results/e86/e86e97ff7887f1bdc6bf913498392ff50f56ea88/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/xz/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_XZ
 	bool "xz-utils"
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
 	  XZ is the successor to the Lempel-Ziv/Markov-chain Algorithm
 	  compression format, which provides memory-hungry but powerful
@@ -12,3 +13,6 @@ config BR2_PACKAGE_XZ
 	  package.
 
 	  http://tukaani.org/xz/
+
+comment "xz-utils needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS