util: Move p_debug.h into util module.

The debug functions depend on several util function for os abstractions, and
these depend on debug functions, so a seperate module is not possible.

src/gallium/auxiliary/pipebuffer/pb_buffer_fenced.c

@@ -43,7 +43,7 @@
 
 #include "pipe/p_compiler.h"
 #include "pipe/p_error.h"
-#include "pipe/p_debug.h"
+#include "util/u_debug.h"
 #include "pipe/p_thread.h"
 #include "util/u_memory.h"
 #include "util/u_double_list.h"