Mark miniDump() as pure virtual.

Fixes a build error when building with -O0 due to missing vtable entry.

Bug: 131121064
Test: make NATIVE_COVERAGE=true
COVERAGE_PATHS=frameworks/native/services/surfaceflinger

Change-Id: Iaefb6921b4fe95a2c138444a0cfba2ecb739550f
Merged-In: Iaefb6921b4fe95a2c138444a0cfba2ecb739550f
(cherry picked from commit f3b7c9c6)