LoongArch: Remove posix_types.h include from sigcontext.h

LoongArch: Remove posix_types.h include from sigcontext.h

Nothing in sigcontext.h seems to require anything from
linux/posix_types.h.  This include seems a MIPS relic originated from
an error in Linux 2.6.11-rc2 (in 2005).

The unneeded include was found debugging some vDSO self test build
failure (it's not the root cause though).

Link: https://lore.kernel.org/linux-mips/20240828030413.143930-2-xry111@xry111.site/
Link: https://lore.kernel.org/loongarch/0b540679ec8cfccec75aeb3463810924f6ff71e6.camel@xry111.site/
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>