mips: Remove posix_types.h include from sigcontext.h

Nothing in sigcontext.h seems to require anything from
linux/posix_types.h.

It seems only a relict: in a Linux 2.6.11-rc2 patch [1] the
linux/types.h include was unexplainedly changed to a linux/posix_types.h
include.  I can only assume it was just an error.  Finally headers_check
complained "found __[us]{8,16,32,64} type without #include
<linux/types.h>" and commit ae612fb0 ("headers_check fix: mips,
sigcontext.h") added back the linux/types.h include, but it didn't
remove the posix_types.h include.

Remove it now.

[1]:https://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.11-rc2/2.6.11-rc2-mm2/broken-out/mips-generic-mips-updates.patch



Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>