CMakeLists: Don't dump xxhash's includes into top-level directory scope

We already use a custom CMakeLists file for xxhash, so we can just make it's headers
public as part of its target interface.

This way, only libraries that link in the xxhash target will see its headers, as opposed
to every target under the top-level directory.

CMakeLists.txt

@@ -566,7 +566,6 @@ endif()
 if(NOT XXHASH_FOUND)
   message(STATUS "Using static xxhash from Externals")
   add_subdirectory(Externals/xxhash)
-  include_directories(Externals/xxhash)
 endif()
 
 find_package(ZLIB)

Externals/xxhash/CMakeLists.txt

@@ -1,7 +1,7 @@
 project(xxhash C)
 
-set(SRCS
-	xxhash.c
+add_library(xxhash STATIC xxhash.c)
+target_include_directories(xxhash
+PUBLIC
+  ${CMAKE_CURRENT_SOURCE_DIR}
 )
-
-add_library(xxhash STATIC ${SRCS})