Fix build for systems without LZ4 dependency installed
Fix build for systems without LZ4 dependency installed

This was introduced by commit 903c91cd3013c763abd3f2292f8fbd911a0c83f0.
To fix it, we simply protect the try_compress_lz4 function with proper
ifdef guards.

Build log:

spicevmc.c: In function 'try_compress_lz4':
spicevmc.c:143:5: error: implicit declaration of function 'LZ4_compress_default' [-Werror=implicit-function-declaration]
     compressed_data_count = LZ4_compress_default((char*)&msg_item->buf,
     ^
spicevmc.c:143:5: error: nested extern declaration of 'LZ4_compress_default' [-Werror=nested-externs]

spicevmc.c: At top level:
spicevmc.c:124:24: error: 'try_compress_lz4' defined but not used [-Werror=unused-function]
 static RedVmcPipeItem* try_compress_lz4(SpiceVmcState *state, int n, RedVmcPipeItem *msg_item)
                        ^

Signed-off-by: Eduardo Lima (Etrunko) <025eb0974a9f48eaca88b86d8c2bcaf361e95a91@redhat.com>
Acked-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice