tls: Include <fcntl.h> instead of <sys/fcntl.h>

musl libc warns about it:
```
In file included from src/tls/cockpit-certificate-ensure.c:32:
/usr/x86_64-pc-linux-musl/include/sys/fcntl.h:1:2: warning: redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-W#warnings]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
      |  ^
```
and it's just a wrapper in glibc as well. Use `<fcntl.h>` to be warning-free
when building against musl

src/tls/cockpit-certificate-ensure.c

@@ -24,11 +24,11 @@
 #include <assert.h>
 #include <err.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <spawn.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/fcntl.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <unistd.h>