[7.6] Fix type mismatch.

gdb/gdb-7.6/gdb/gdbserver/thread-db.c: In function 'thread_db_load_search':
gdb/gdb-7.6/gdb/gdbserver/thread-db.c:561:30: error:
assignment from incompatible pointer type [-Werror]

Change-Id: I4471b99aa2b6d2804bcac3e198c0a972d36d75a3