staging: android: ashmem: Declared file operation with const keyword

Warning found by checkpatch.pl script

Signed-off-by: Thais Camacho <thaiscamachoo@gmail.com>

drivers/staging/android/ashmem.c

@@ -376,7 +376,7 @@ ashmem_vmfile_get_unmapped_area(struct file *file, unsigned long addr,
 
 static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	static struct file_operations vmfile_fops;
+	static const struct file_operations vmfile_fops;
 	struct ashmem_area *asma = file->private_data;
 	int ret = 0;