macOS: unix-utils: Include <sys/mount.h>.

If <sys/mount.h> is available on the platform, always include
it in this file.

common/mlutils/unix_utils-c.c

@@ -55,6 +55,10 @@
 #include <windows.h>
 #endif
 
+#ifdef HAVE_SYS_MOUNT_H
+#include <sys/mount.h>
+#endif
+
 #include <caml/alloc.h>
 #include <caml/fail.h>
 #include <caml/memory.h>

m4/guestfs-libraries.m4

@@ -47,6 +47,7 @@ AC_CHECK_HEADERS([\
     linux/rtc.h \
     printf.h \
     sys/inotify.h \
+    sys/mount.h \
     sys/resource.h \
     sys/socket.h \
     sys/statfs.h \