added prefix to header macros

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

src/dirvec.h

@@ -1,5 +1,5 @@
-#ifndef DIRVEC_H
-#define DIRVEC_H
+#ifndef MPD_DIRVEC_H
+#define MPD_DIRVEC_H
 
 #include <stddef.h>