util: Created header to define functions that may be used in more that one place

Using local function prototype redefinition is dangerous, now the
prototype is seen in all concerned files.

util/setstyle.c

@@ -38,6 +38,8 @@
 
 #include "../src/wconfig.h"
 
+#include "common.h"
+
 #define MAX_OPTIONS 128
 
 char *FontOptions[] = {
@@ -75,7 +77,6 @@ int ignoreCursors = 0;
 
 Display *dpy;
 
-extern char *convertFont(char *font, Bool keepXLFD);
 
 static Bool isCursorOption(char *option)
 {