cmake: fix def propagation, fixes 32-bit build

32-bit build would fail to link with 'mdb_env_create undefined' (because
for 32-bit build, mdb_env_create_vl32 is defined instead).

This bug was introduced with the recent change to virtual object
libraries. The problem is that the COMPILE_DEFINITIONS property
was not propagated from dependee target (lmdb) to depedent
target's (blockchain_db) virtual object lib (obj_blockchain_db).
This patch makes that happen.

I chose to include INTERFACE_COMPILE_DEFINITIONS because there
should not be a need to propagate private defs, but it doesn't
make a difference in this case.

src/CMakeLists.txt

@@ -92,6 +92,8 @@ function (bitmonero_add_library name)
   set_property(TARGET "${name}"
     PROPERTY
       FOLDER "libs")
+  target_compile_definitions(${objlib}
+    PRIVATE $<TARGET_PROPERTY:${name},INTERFACE_COMPILE_DEFINITIONS>)
 endfunction ()
 
 add_subdirectory(common)