Use #include <unistd.h> for obtaining the prototype of write()

[AdrianBunk]

Commit 5823762 did

-	extern int write();\
+	extern int write(int, const void *, unsigned);\

but the actual function signature is

ssize_t write (int, const void *, size_t);

The 80s are over, just include the header for the prototype.

@bmc @markyang92

[vjardin]

LGTM too.

Please, @bmc : when can you merge it ?

If I may allow, you should rever the #37 and apply this fix which is much better.