mkimage: fit: Unmmap the memory before closing fd in fit_import_data()

Without calling munmap(), the follow-up call to open() the same file
with a flag O_TRUNC seems not to cause any issue on Linux, but it fails
on Windows with error like below:

    Can't open kernel_fdt.itb.tmp: Permission denied

Fix this by unmapping the memory before closing fd in fit_import_data().

Signed-off-by: Lihua Zhao <lihua.zhao@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>