[NaCl SDK] Remove sys/signal.h from newlib-extras

The toolchains now contain an up-to-date version
of thie header to its not longer needed here.

Review URL: https://codereview.chromium.org/666163004

Cr-Commit-Position: refs/heads/master@{#300534}

native_client_sdk/src/libraries/nacl_io/library.dsc

@@ -232,7 +232,6 @@
         "sys/mount.h",
         "sys/poll.h",
         "sys/select.h",
-        "sys/signal.h",
         "sys/socket.h",
         "sys/termios.h",
         "sys/time.h",