[AArch64] Fix incorrectly-defined SVE macro

[AArch64] Fix incorrectly-defined SVE macro

The kernel has fixed this here:

https://lore.kernel.org/patchwork/patch/1029011/

We should do the same for GDB, which is still carrying an incorrect
definition of the macro.  As stated in the kernel patch thread, this doesn't
actually change things because, luckily, the structs are of the same size.

gdb/ChangeLog:

2020-08-10  Luis Machado  <luis.machado@linaro.org>

	* nat/aarch64-sve-linux-sigcontext.h (SVE_PT_REGS_OFFSET): Use
	struct user_sve_header instead of struct sve_context.