lib/yajl: drop "yajl/" prefix from #include paths

According to the yajl API documentation, #include lines should have
the "yajl/" path prefix, but the actual pkg-config file contains:

 includedir=${dollar}{prefix}/include/yajl

.. which already contains this directory name, and thus the "yajl/"
prefix cannot work.  Unfortunately, the yajl project hasn't been
maintained for nearly 10 years, and there's little chance this bug
will ever be fixed.

src/lib/yajl/Callbacks.hxx

@@ -27,13 +27,12 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef YAJL_CALLBACKS_HXX
-#define YAJL_CALLBACKS_HXX
+#pragma once
 
 #include "util/Cast.hxx"
 #include "util/StringView.hxx"
 
-#include <yajl/yajl_parse.h>
+#include <yajl_parse.h>
 
 namespace Yajl {
 
@@ -81,5 +80,3 @@ struct CallbacksWrapper {
 };
 
 } // namespace Yajl
-
-#endif

src/lib/yajl/Gen.hxx

@@ -27,12 +27,11 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef YAJL_GEN_HXX
-#define YAJL_GEN_HXX
+#pragma once
 
 #include "util/ConstBuffer.hxx"
 
-#include <yajl/yajl_gen.h>
+#include <yajl_gen.h>
 
 #include <algorithm>
 #include <string_view>
@@ -101,5 +100,3 @@ public:
 };
 
 } // namespace Yajl
-
-#endif

src/lib/yajl/Handle.hxx

@@ -27,10 +27,9 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef YAJL_HANDLE_HXX
-#define YAJL_HANDLE_HXX
+#pragma once
 
-#include <yajl/yajl_parse.h>
+#include <yajl_parse.h>
 
 #include <utility>
 
@@ -82,5 +81,3 @@ private:
 };
 
 } // namespace Yajl
-
-#endif