hw/mem/meson: Fix linking sparse-mem device with fuzzer

hw/mem/meson: Fix linking sparse-mem device with fuzzer

sparse-mem.c is added to the 'mem_ss' source set, which itself
is conditionally added to softmmu_ss if CONFIG_MEM_DEVICE is
selected.
But if CONFIG_MEM_DEVICE isn't selected, we get a link failure
even if CONFIG_FUZZ is selected:

  /usr/bin/ld: tests_qtest_fuzz_generic_fuzz.c.o: in function `generic_pre_fuzz':
  tests/qtest/fuzz/generic_fuzz.c:826: undefined reference to `sparse_mem_init'
  clang-10: error: linker command failed with exit code 1 (use -v to see invocation)

Fix by adding sparse-mem.c directly to the softmmu_ss set.

Fixes: 230376d285b ("memory: add a sparse memory device for fuzzing")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20210406133944.4193691-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>