Fixed a compilation warning in mlx_new_image.c file

[ismaelmehdid]

This pull request addresses a compilation warning in mlx_new_image.c related to the shm_att_pb function.
The warning indicated that the return value of the write function was being ignored : warn_unused_result.

Changes Made:

Previously, the code had a line where write(2, WARN_SHM_ATTACH, strlen(WARN_SHM_ATTACH)) was called without handling its return value.
Now the code properly handle the return value of the write function.
The modification ensures that the return value is returned by the shm_att_pb function.

[dimadem]

Thank you. I'm using your solution, in my case it worked.