Fix (most) warnings.

There is still a signed/unsigned warning for the preset argument to lzma_lzma_preset().
diff --git a/lib/zip_algorithm_xz.c b/lib/zip_algorithm_xz.c
index bb734f4..16f081a 100644
--- a/lib/zip_algorithm_xz.c
+++ b/lib/zip_algorithm_xz.c
@@ -89,12 +89,12 @@
 
 static int
 compression_flags(void *ud) {
-    struct ctx *ctx = (struct ctx *)ud;
+    /* struct ctx *ctx = (struct ctx *)ud; */
     return 0;
 }
 
 static int
-map_error(int ret) {
+map_error(lzma_ret ret) {
     switch (ret) {
       case LZMA_UNSUPPORTED_CHECK:
         return ZIP_ER_COMPRESSED_DATA;
@@ -152,7 +152,6 @@
 static bool
 end(void *ud) {
     struct ctx *ctx = (struct ctx *)ud;
-    int err;
 
     lzma_end(&ctx->zstr);
     return true;