Fix memory leak

[messmerd]

In interface.cc, an array of Descriptor objects is dynamically created and then later destroyed by calling delete on pointers to the base class DescriptorStub. However, since DescriptorStub's destructor is non-virtual, the destructors of derived classes are not called and this causes a memory leak which I detected with AddressSanitizer:

==1814137==ERROR: AddressSanitizer: new-delete-type-mismatch on 0x60e0001424a0 in thread T0:
  object passed to delete has wrong type:
  size of the allocated type:   160 bytes;
  size of the deallocated type: 152 bytes.
    #0 0x7f15d641822f in operator delete(void*, unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:172
    #1 0x7f15c5510d3c in caps_so_fini /home/dalton/Documents/lmms_dev2/plugins/LadspaEffect/caps/interface.cc:132
    #2 0x7f15d6d6e24d in _dl_fini elf/dl-fini.c:142
    #3 0x7f15d370a494 in __run_exit_handlers stdlib/exit.c:113
    #4 0x7f15d370a60f in __GI_exit stdlib/exit.c:143
    #5 0x7f15d36eed96 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:74
    #6 0x7f15d36eee3f in __libc_start_main_impl ../csu/libc-start.c:392
    #7 0x55982aedfc34 in _start (/home/dalton/Documents/lmms_dev2/build/lmms+0x798c34)

0x60e0001424a0 is located 0 bytes inside of 160-byte region [0x60e0001424a0,0x60e000142540)
allocated by thread T0 here:
    #0 0x7f15d64171c7 in operator new(unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f15c55110a4 in caps_so_init /home/dalton/Documents/lmms_dev2/plugins/LadspaEffect/caps/interface.cc:77

SUMMARY: AddressSanitizer: new-delete-type-mismatch ../../../../src/libsanitizer/asan/asan_new_delete.cpp:172 in operator delete(void*, unsigned long)
==1814137==HINT: if you don't care about these errors you may set ASAN_OPTIONS=new_delete_type_mismatch=0
==1814137==ABORTING

This PR fixes the problem by making DescriptorStub's destructor virtual.