Fix warnings for -Wmissing-prototypes

Add -Wmissing-prototypes if compiling with gcc
Static where possible

git-svn-id: https://svn.musicpd.org/mpd/trunk@4657 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/volume.h

@@ -27,13 +27,13 @@
 #define VOLUME_MIXER_ALSA	"alsa"
 #define VOLUME_MIXER_SOFTWARE	"software"
 
-void initVolume();
+void initVolume(void);
 
-void openVolumeDevice();
+void openVolumeDevice(void);
 
-void finishVolume();
+void finishVolume(void);
 
-int getVolumeLevel();
+int getVolumeLevel(void);
 
 int changeVolumeLevel(int fd, int change, int rel);