fix gettid() declaration

See github bug report #16.

include/unistd.h

@@ -1260,7 +1260,7 @@ extern int getentropy(void *__buf, size_t __len) __nonnull ((1)) __wur;
 #endif
 
 #if (defined _GNU_SOURCE && defined __UCLIBC_HAS_THREADS_NATIVE__)
-pid_t gettid(void);
+extern __pid_t gettid(void);
 #endif
 
 __END_DECLS