Include signal.h in nat/amd64-linux-siginfo.h

Include signal.h in nat/amd64-linux-siginfo.h

$ make check-headers CHECK_HEADERS="nat/amd64-linux-siginfo.h"
....
../../binutils-gdb/gdb/nat/amd64-linux-siginfo.h:52:39: error: 'siginfo_t' was not declared in this scope
 int amd64_linux_siginfo_fixup_common (siginfo_t *native, gdb_byte *inf,
                                       ^
gdb:

2017-01-06  Yao Qi  <yao.qi@linaro.org>

	* nat/amd64-linux-siginfo.h: Include signal.h.