memory_hook: Default virtual destructor in the cpp file

Prevents creating multiple copies of the vtable in every translation unit that uses the class.
Also silences a -Wweak-vtables warning

src/core/CMakeLists.txt

@@ -253,6 +253,7 @@ add_library(core STATIC
     loader/nso.h
     memory.cpp
     memory.h
+    memory_hook.cpp
     memory_hook.h
     memory_setup.h
     perf_stats.cpp