spl: Fix compiling warning on gunzip argument

common/spl/spl_fit.c:201:12: warning: passing argument 4 of ‘gunzip’
from incompatible pointer type [-Wincompatible-pointer-types]
       src, &length))

Signed-off-by: York Sun <york.sun@nxp.com>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
CC: Jean-Jacques Hiblot <jjhiblot@ti.com>