Fix Native Plugin Loading for CMake

[thestr4ng3r]

Regression introduced by #1805, cc @karliss:

[florian@florian-desktop cmake-build-debug]$ ./Cutter
Loading "/home/florian/.cutterrc"
Plugins are loaded from "/home/florian/.local/share/RadareOrg/Cutter/plugins"
Load Error for plugin "libr2ghidra_cutter.so" : "Cannot load library /home/florian/.local/share/RadareOrg/Cutter/plugins/native/libr2ghidra_cutter.so: (/home/florian/.local/share/RadareOrg/Cutter/plugins/native/libr2ghidra_cutter.so: undefined symbol: _ZNK10Decompiler10metaObjectEv)"
Loaded 0 plugin(s).

Because of https://cmake.org/cmake/help/v3.4/policy/CMP0065.html#policy:CMP0065, which was introduced in CMake 3.4, ENABLE_EXPORTS was disabled by default, which is required for native plugins to work.