btrfs-progs: build: use plain fcntl.h instead of sys/fcntl.h

The CI build on musl warns:

    [CC]     cmds/balance.o
In file included from cmds/inspect.c:19:
/usr/include/sys/fcntl.h:1:2: warning: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
      |  ^~~~~~~

On glibc the header directly includes fcntl.h.

Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-sb-mod.c

@@ -15,8 +15,8 @@
  */
 
 #include "kerncompat.h"
-#include <sys/fcntl.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>

cmds/inspect.c

@@ -16,11 +16,11 @@
 
 #include "kerncompat.h"
 #include <sys/ioctl.h>
-#include <sys/fcntl.h>
 #include <sys/stat.h>
 #include <sys/statfs.h>
 #include <linux/fs.h>
 #include <linux/magic.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>

cmds/reflink.c

@@ -15,8 +15,8 @@
  */
 
 #include "kerncompat.h"
-#include <sys/fcntl.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <getopt.h>
 #include <unistd.h>