readerfactory.c: do not #include <alloca.h>

Compilation fails on NetBSD with:
../src/readerfactory.c:52:10: fatal error: alloca.h: No such file or directory
 #include <alloca.h>
          ^~~~~~~~~~

alloca(3) is defined in stdlib.h on NetBSD.

src/readerfactory.c

@@ -49,7 +49,6 @@ THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <errno.h>
 #include <fcntl.h>
 #include <pthread.h>
-#include <alloca.h>
 #include <stdatomic.h>
 #include <stdbool.h>