make galois_init_default_field(int w) extern

make galois_init_default_field(int w) extern

So that the application can initialize gfp_array and gfp_is_composite
instead of relying on initialization happening implicitly when the
multiply or xor functions are called. The init function can be called
once when the application guarantees thread safety. And the multiply and
xor functions can be called from multiple threads without risking races.

Signed-off-by: Loic Dachary <loic@dachary.org>