Fix warning in musl related to ioctl

Ioctl's in musl take ints, while glibc take unsigned longs. When not
using glibc, default to casting the ioctl number to an int. Tests
passing.

writeimg.c

@@ -278,7 +278,11 @@ int main(int argc, char *argv[]) {
             exit(EXIT_FAILURE);
         }
 
+#ifdef __GLIBC__
         if (ioctl(fd, BLKGETSIZE64, &device_size) < 0) {
+#else /* With musl, ioctl's take ints, kheaders provide unsigned longs. Passes tests. */
+        if (ioctl(fd, (int)BLKGETSIZE64, &device_size) < 0) {
+#endif
             perror("ioctl");
             close(fd);
             exit(EXIT_FAILURE);