Add prototypes for vwarn/vwarnx

Fixes this warning:
common.c:406:1: warning: no previous prototype for 'vwarn' [-Wmissing-prototypes]
  406 | vwarn(const char *fmt, va_list list)
      | ^~~~~
common.c:429:1: warning: no previous prototype for 'vwarnx' [-Wmissing-prototypes]
  429 | vwarnx(const char *fmt, va_list list)
      | ^~~~~~

Also move err() to have non-x versions first.

src/common.c

@@ -425,6 +425,17 @@ warn(const char *fmt, ...)
 	va_end(list);
 }
 
+void
+err(int eval, const char *fmt, ...)
+{
+	va_list list;
+
+	va_start(list, fmt);
+	vwarn(fmt, list);
+	va_end(list);
+	exit(eval);
+}
+
 void
 vwarnx(const char *fmt, va_list list)
 {
@@ -442,17 +453,6 @@ warnx(const char *fmt, ...)
 	va_end(list);
 }
 
-void
-err(int eval, const char *fmt, ...)
-{
-	va_list list;
-
-	va_start(list, fmt);
-	vwarn(fmt, list);
-	va_end(list);
-	exit(eval);
-}
-
 void
 errx(int eval, const char *fmt, ...)
 {

src/common.h

@@ -132,10 +132,12 @@ int check_topdomain(char *, char **);
 int inet_aton(const char *cp, struct in_addr *inp);
 #endif
 
-void err(int eval, const char *fmt, ...);
+void vwarn(const char *fmt, va_list list);
 void warn(const char *fmt, ...);
-void errx(int eval, const char *fmt, ...);
+void err(int eval, const char *fmt, ...);
+void vwarnx(const char *fmt, va_list list);
 void warnx(const char *fmt, ...);
+void errx(int eval, const char *fmt, ...);
 #endif
 
 int recent_seqno(int , int);