[libunwind] include alloca.h in test/aarch64_za_unwind.pass.cpp (#207376)

Glibc provides the alloca function as a part of stdlib.h when
_GNU_SOURCE is declared (which is the case with standard linux builds).
[1]

On non-glibc systems, this may not be the case (for example, picolibc).
These libraries may not implicitly include alloca.h as a part of the
standard lib.

[1]
https://github.com/bminor/glibc/blob/04e750e75b73957cf1c791535a3f4319534a52fc/stdlib/stdlib.h#L728

Signed-off-by: Shreeyash Pandey <shrpand@qti.qualcomm.com>
GitOrigin-RevId: 6eb35326147077a18f6fc56540059d779d7e3d4b