Add sys/file.h for LOCK_

Fixes build with musl:
| ../git/src/shared/dissect-image.c: In function 'mount_image_privately_interactively':
| ../git/src/shared/dissect-image.c:2986:34: error: 'LOCK_SH' undeclared (first use in this function)
|  2986 |         r = loop_device_flock(d, LOCK_SH);
|       |                                  ^~~~~~~

src/test/test-loop-block.c

@@ -3,6 +3,7 @@
 #include <fcntl.h>
 #include <linux/loop.h>
 #include <pthread.h>
+#include <sys/file.h>
 
 #include "alloc-util.h"
 #include "dissect-image.h"