libmpv: use correct header directory name

libmpv headers are installed to `mpv/`, so why are we pretending it is
`libmpv`?

Fixes documentation, libmpv meson dependency, tests.

misc/json.c

@@ -48,7 +48,7 @@
 #include <inttypes.h>
 #include <assert.h>
 
-#include <libmpv/client.h>
+#include <mpv/client.h>
 
 #include "common/common.h"
 #include "misc/bstr.h"

misc/node.c

@@ -1,6 +1,6 @@
 #include "node.h"
 
-#include <libmpv/client.h>
+#include <mpv/client.h>
 
 #include "common/common.h"
 #include "bstr.h"