libavb_user: convert avb_ops_user.c to .cpp

fs_mgr_get_entry_for_mount_point() is switching to a C++ only API (using
std::string). Do a minimum conversion in libavb_user to adopt that
change.

Bug: 67024518
Test: avbctl get-verification
Test: avbctl disable-verification
Test: avbctl enable-verification
Test: All unit tests pass

Change-Id: I755d86d517409ba00020708784854416b3c7b2bd
diff --git a/Android.bp b/Android.bp
index a512a22..81e3f8a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -116,7 +116,7 @@
     srcs: [
         "libavb/avb_sysdeps_posix.c",
         "libavb_ab/avb_ab_flow.c",
-        "libavb_user/avb_ops_user.c",
+        "libavb_user/avb_ops_user.cpp",
         "libavb_user/avb_user_verity.c",
         "libavb_user/avb_user_verification.c",
     ],