HACK: ztest: fix redefintion of nano_fifo_get()

ztest code redefines this as C function, which causes problems when
it appears in the legacy.h header as a static inline function.

Change-Id: I3ed9fbece3a304f706857a16bdca8dfb11a7802c
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>