hplip: needs threads

Hplip needs threads because libusb needs them. Fixes:
http://autobuild.buildroot.net/results/12fb9bb8c93c1cc1f93ba8e15558d1630f2e7c3d/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/hplip/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_HPLIP
 	bool "hplip"
 	depends on BR2_INSTALL_LIBSTDCPP
 	depends on BR2_PACKAGE_CUPS
+	depends on BR2_TOOLCHAIN_HAS_THREADS # libusb
 	select BR2_PACKAGE_LIBUSB
 	select BR2_PACKAGE_JPEG
 	help
@@ -14,5 +15,5 @@ config BR2_PACKAGE_HPLIP
 
 	  http://hplipopensource.com/
 
-comment "hplip requires a toolchain with C++ support enabled"
-	depends on !BR2_INSTALL_LIBSTDCPP
+comment "hplip requires a toolchain with C++ and thread support enabled"
+	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS