build: Allow using system xxhash

meson.build

@@ -2268,6 +2268,7 @@ util_ss.add_all(trace_ss)
 util_ss = util_ss.apply(config_all, strict: false)
 libqemuutil = static_library('qemuutil',
                              sources: util_ss.sources() + stub_ss.sources() + genh,
+                             include_directories: 'util/xxHash',
                              dependencies: [util_ss.dependencies(), libm, threads, glib, socket, malloc, pixman])
 qemuutil = declare_dependency(link_with: libqemuutil,
                               sources: genh + version_res)

util/fast-hash.c

@@ -1,5 +1,5 @@
 #include "qemu/fast-hash.h"
-#include "util/xxHash/xxh3.h"
+#include <xxh3.h>
 
 uint64_t fast_hash(const uint8_t *data, size_t len)
 {