selftests: mptcp: Mark xerror and die_perror __noreturn

selftests: mptcp: Mark xerror and die_perror __noreturn

Compiler reports potential uses of uninitialized variables in
mptcp_connect.c when xerror() is called from failure paths.

mptcp_connect.c:1262:11: warning: variable 'raw_addr' is used
      uninitialized whenever 'if' condition is false
      [-Wsometimes-uninitialized]

xerror() terminates execution by calling exit(), but it is not visible
to the compiler & assumes control flow may continue past the call.

Annotate xerror() with __noreturn so the compiler can correctly reason
about control flow and avoid false-positive uninitialized variable
warnings.

Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Link: https://patch.msgid.link/20260101172840.90186-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>