CMake: fix lifflefs filesystem header include path

- LittleFileSystem.h and LittleFileSystem.cpp refers to the littflefs headers with the complete path like storage/filesystem/littlefs/littlefs/lfs_util.h and fixed the path issue

Author: rajkan01

storage/filesystem/littlefs/include/littlefs/LittleFileSystem.h

@@ -24,7 +24,7 @@
 #include "filesystem/FileSystem.h"
 #include "blockdevice/BlockDevice.h"
 #include "platform/PlatformMutex.h"
-#include "storage/filesystem/littlefs/littlefs/lfs.h"
+#include "littlefs/lfs.h"
 
 namespace mbed {
 

storage/filesystem/littlefs/source/LittleFileSystem.cpp

@@ -17,8 +17,8 @@
 #include "filesystem/mbed_filesystem.h"
 #include "littlefs/LittleFileSystem.h"
 #include "errno.h"
-#include "storage/filesystem/littlefs/littlefs/lfs.h"
-#include "storage/filesystem/littlefs/littlefs/lfs_util.h"
+#include "littlefs/lfs.h"
+#include "littlefs/lfs_util.h"
 #include "MbedCRC.h"
 
 namespace mbed {