Modifications to run microROS on NuttX RTOS

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## Issue template
microROS on NuttX target runs correctly again, but there are some corrections required to correctly compile the example.

- Hardware description: nucelo-144 board
- RTOS: NuttX
- Installation type: microROS for NuttX (this example)
- Version or commit hash: galactic

#### Steps to reproduce the issue

The file micro_ros_src/src/rcutils/src/filesystem.c contains a redefinition of "DIR" (which is already defined in the dirent.h" file in NuTTX).

A test is required such

#ifdef DIR or #ifndef DIR....

#### Expected behavior

the nuttx application compile again

#### Actual behavior

There is the error about redefining DIR. To get the compilaton correctly I've commented the line

#ifdef RCUTILS_NO_FILESYSTEM
//typedef int DIR; <=======
#endif // _RCUTILS_NO_FILESYSTEM
typedef struct rcutils_dir_iter_state_t
{
#ifdef _WIN32
HANDLE handle;
WIN32_FIND_DATA data;
#else
DIR * dir;
#endif
} rcutils_dir_iter_state_t;

but it is not the correct solution.

#### Additional information

I've modified also the microros_main.c file in the microros/example_app

Start by reproducing the NuttX application compilation and inspect micro_ros_src/src/rcutils/src/filesystem.c alongside NuttX's dirent.h definition of DIR. Check the conditional handling around RCUTILS_NO_FILESYSTEM and verify the example compiles without redefining DIR; the issue also mentions changes in microros/example_app/microros_main.c, which may need review.

Written by the indexing model from the issue text.