external toolchain: also copy the libthread_db.so for gdbserver

gdbserver dlopen(3)s libthread_db.so at runtime, so there is no
dependency on it (does not appear as being (NEEDED)).

Copy libthread_db.so from external toolchain when gdbserver is enbled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/external-toolchain/ext-tool.mk

@@ -264,6 +264,9 @@ endif
 
 ifneq ($(BR2_PTHREADS_NONE),y)
 EXTERNAL_LIBS+=libpthread.so
+ifeq ($(BR2_PACKAGE_GDB_SERVER),y)
+EXTERNAL_LIBS+=libthread_db.so
+endif # gdbserver
 endif # ! no threads
 
 # SYSROOT_DIR selection. We first try the -print-sysroot option,