xtensa: uapi: Reuse asm-generic ucontext.h

Drop the custom non-uapi definition which is the same as the generic
uapi one.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Message-ID: <20260314-uapi-ucontext-xtensa-v1-1-08dceca7e6a2@weissschuh.net>
[Max: Move new generic-y line to the arch/xtensa/include/uapi/asm/Kbuild]
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Author: t-8ch

arch/xtensa/include/asm/ucontext.h

@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 generated-y += unistd_32.h
+generic-y += ucontext.h