pragma once for header file

Add missing "#pragma once" to header file.

Bug: none
Test: build
Change-Id: I9ea248686dc2affc8ba093c58a2660505f5cc8de
Signed-off-by: Alessio Balsini <balsini@google.com>

fs_mgr/libsnapshot/dm_snapshot_internals.h

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#pragma once
+
 #include <stdint.h>
 
 #include <vector>