Fix issues found with -Wstrict-prototypes

https://github.com/datajerk/c2d/pull/2
--- c2d.c
+++ c2d.c
@@ -49,7 +49,7 @@ Bugs:
 #define MONITOR 1
 #define LOADER "loader"
 
-void usage();
+void usage(void);
 char *getext(char *filename);
 
 int main(int argc, char **argv)
@@ -346,7 +346,7 @@ char *getext(char *filename)
 	return (rval);
 }
 
-void usage()
+void usage(void)
 {
 	fprintf(stderr, "%s", usagetext);
 }
--- page2text.c
+++ page2text.c
@@ -2,7 +2,7 @@
 
 #define MASK 0x7F
 
-int main()
+int main(int argc, char **argv)
 {
 	int i, j, k, line;
 	char screen[24][40];
--- text2page.c
+++ text2page.c
@@ -3,7 +3,7 @@
 #define NORMAL 0x80
 #define BLINK 0x40
 
-int main()
+int main(int argc, char **argv)
 {
 	char c;
 	int i, j, k, line = 0;
