env_callback.h: spl: mark callback as maybe_unused

When static inline is used in a header file the function
should preferably be inlined and if not possible made a
static function. When declared inside a c file there is a
static function, which might be inlined. Since SPL uses a
define to declare the static inline it becomes part of the
c file although it is declared in a header and clang will
warn that you have introduced unused static functions. Add
maybe_unused to prevent such warnings.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>