compress: fix gcc warnings about void* used in arithmetic

compress: fix gcc warnings about void* used in arithmetic

src/journal/compress.c: In function ‘compress_blob_lz4’:
src/journal/compress.c:115:49: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
         r = LZ4_compress_limitedOutput(src, dst + 8, src_size, (int) dst_alloc_size - 8);
                                                 ^
src/journal/compress.c: In function ‘decompress_blob_xz’:
src/journal/compress.c:179:35: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
                 s.next_out = *dst + used;
                                   ^
src/journal/compress.c: In function ‘decompress_blob_lz4’:
src/journal/compress.c:218:37: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
         r = LZ4_decompress_safe(src + 8, out, src_size - 8, size);
                                     ^
src/journal/compress.c: In function ‘decompress_startswith_xz’:
src/journal/compress.c:294:38: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
                 s.next_out = *buffer + *buffer_size - s.avail_out;
                                      ^
src/journal/compress.c:294:53: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
                 s.next_out = *buffer + *buffer_size - s.avail_out;
                                                     ^
src/journal/compress.c: In function ‘decompress_startswith_lz4’:
src/journal/compress.c:327:45: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
         r = LZ4_decompress_safe_partial(src + 8, *buffer, src_size - 8,
                                             ^

LZ4 and XZ functions use char* and unsigned char*, respectively,
so keep void* in our internal APIs and add casts.