Add missing include for macOS High Sierra build.

See #50.
Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/sfinputstream.cc

@@ -20,6 +20,7 @@
 #include <assert.h>
 #include <string.h>
 #include <unistd.h>
+#include <errno.h>
 
 using std::string;
 using std::vector;