Ensure NULL is defined when players are disabled.

xmpplayer.c fails building when WITH_XMP is disabled from NULL not being
defined.

Instead of including string.h, prefer including only stddef.h since it
contains much less.

Author: FtZPetruska

prboom2/src/MUSIC/flplayer.c

@@ -35,7 +35,8 @@
 #include "musicplayer.h"
 
 #ifndef HAVE_LIBFLUIDSYNTH
-#include <string.h>
+
+#include <stddef.h>
 
 static const char *fl_name (void)
 {

prboom2/src/MUSIC/madplayer.c

@@ -37,7 +37,8 @@
 #include "musicplayer.h"
 
 #ifndef HAVE_LIBMAD
-#include <string.h>
+
+#include <stddef.h>
 
 static const char *mp_name (void)
 {

prboom2/src/MUSIC/portmidiplayer.c

@@ -38,7 +38,8 @@
 #include "musicplayer.h"
 
 #ifndef HAVE_LIBPORTMIDI
-#include <string.h>
+
+#include <stddef.h>
 
 static const char *pm_name (void)
 {

prboom2/src/MUSIC/vorbisplayer.c

@@ -37,7 +37,8 @@
 #include "musicplayer.h"
 
 #ifndef HAVE_LIBVORBISFILE
-#include <string.h>
+
+#include <stddef.h>
 
 static const char *vorb_name (void)
 {

prboom2/src/MUSIC/xmpplayer.c

@@ -22,6 +22,8 @@
 
 #ifndef HAVE_LIBXMP
 
+#include <stddef.h>
+
 static const char *xmp_name(void)
 {
   return "libxmp tracker player (DISABLED)";