target gdb: depend on thread support

Compiling gdb for the target requires thread support in the C library,
otherwise:

/home/test/outputs/test-888/toolchain/gdb-7.3.1/gdb/gdb_thread_db.h:37:21: fatal error: pthread.h: No such file or directory

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/gdb/Config.in

@@ -4,6 +4,7 @@ config BR2_PACKAGE_GDB
 	bool "Build gdb debugger for the Target"
 	select BR2_PACKAGE_NCURSES
 	depends on BR2_USE_WCHAR
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on !BR2_sh && !BR2_sh64
 	help
 	    Build the full gdb debugger to run on the target.