MLK-20239-1 imx8m: Fix resource leek

Fix coverity issu CID 4338241: Resource leak (RESOURCE_LEAK)
leaked_storage: Variable file_ptr going out of scope leaks the storage it points to.

Add munmap for the file_ptr
Signed-off-by: Ye Li <ye.li@nxp.com>

iMX8M/mkimage_imx8.c

@@ -840,6 +840,7 @@ void generate_sld_with_ivt(char * input_file, uint32_t ep, char *out_file)
 		exit(EXIT_FAILURE);
 	}
 
+	munmap((void *)file_ptr, sbuf.st_size);
 	close(ivt_fd);
 	close(input_fd);
 }