libsnapshot: Add diagnostics for DM_DEV_REMOVE failures.

Example log line:

    update_engine: Block device was lazily unmounted and is still in-use:
        /dev/block/dm-28; possibly open file descriptor or attached loop device.

This will help diagnose bugs such as b/184715543 in the future.

Bug: N/A
Test: manual test
Change-Id: Ia6b17fe9bd1796d59be7fc0b355218509acfd4af
diff --git a/fastboot/Android.bp b/fastboot/Android.bp
index 43b2ddd..ce702a0 100644
--- a/fastboot/Android.bp
+++ b/fastboot/Android.bp
@@ -183,6 +183,7 @@
     ],
 
     static_libs: [
+        "libc++fs",
         "libgtest_prod",
         "libhealthhalutils",
         "libsnapshot_cow",