linux-generic: add header guard for <not-cancel.h>

linux-generic: add header guard for <not-cancel.h>

In general we just try to include <not-cancel.h> once, thus avoiding
any issues with multiple inclusion.

However, the getaddrinfo sources are somewhat tangled, and .c files
include each other to get the final result.  Each .c file currently has
its own include of <not-cancel.h>.  While you could properly re-factor
this for the case of gai.c, the posix/tst-rfc3484.c tests directly include
one of the sub-files (sysdeps/posix/getaddrinfo.c), and it starts to
feel fragile.