Bump xz requirement to 5.2.

5.1.0 does not have proper cmake support.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2747ae6..a054c41 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -221,7 +221,7 @@
 endif(ENABLE_BZIP2)
 
 if(ENABLE_LZMA)
-  find_package(LibLZMA)
+  find_package(LibLZMA 5.2)
   if(LIBLZMA_FOUND)
     set(HAVE_LIBLZMA 1)
   else()