Including ucontext.h doesn't pull in register_t type

[Sean Bruno]

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Test builds of devel/libsigsegv for mips64 seem to fail as the include
files don't seem to line up correctly.  It looks like the standard
ucontext.h doesn't seem to pull in the register_t type so that
machine/ucontext.h can be included.  This seems like a bug somehow, but
I'm not quite sure what the least damaging way of solving it should be.

I'll look at adding _types.h to somewhere in the port include chain, but
I thought I'd at least mention this here.

===========================================================================
=======================<phase: build          >============================
===>  Building for libsigsegv-2.10_1
/nxb-bin/usr/bin/make TARGETSTACK=" all" all-recursive
Making all in src
/bin/sh ../libtool --tag=CC    --mode=compile /nxb-bin/usr/bin/cc
-DHAVE_CONFIG_H -I. -I.. -I. -I.     -O -pipe -G0  -fno-strict-aliasing
-c -o handler.lo handler.c
libtool: compile:  /nxb-bin/usr/bin/cc -DHAVE_CONFIG_H -I. -I.. -I. -I.
-O -pipe -G0 -fno-strict-aliasing -c handler.c  -fPIC -DPIC -o
.libs/handler.o
In file included from /usr/include/ucontext.h:35,
                 from sigsegv.h:21,
                 from handler-unix.c:22,
                 from handler.c:20:
/usr/include/machine/ucontext.h:53: error: expected
specifier-qualifier-list before 'register_t'
*** Error code 1

Stop.
make[3]: stopped in
/wrkdirs/usr/ports/devel/libsigsegv/work/libsigsegv-2.10/src
*** Error code 1
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