[lldb][Posix] Remove unused includes in file system (llvm#121913)

You could remove unistd.h and it will still build, but only because
something else included it. So I've left it in in the spirit of "include
what you use".

Tested on Linux and FreeBSD.

lldb/source/Host/posix/FileSystemPosix.cpp

@@ -9,12 +9,7 @@
 #include "lldb/Host/FileSystem.h"
 
 // C includes
-#include <dirent.h>
 #include <fcntl.h>
-#include <sys/mount.h>
-#include <sys/param.h>
-#include <sys/stat.h>
-#include <sys/types.h>
 #include <unistd.h>
 #if defined(__NetBSD__)
 #include <sys/statvfs.h>