fix make: multiple definition of nrow and ncol

make fail by the error

/usr/bin/ld: main.o:(.bss+0x0): multiple definition of `nrow'; memory.o:(.bss+0x0): first defined here
/usr/bin/ld: main.o:(.bss+0x4): multiple definition of `ncol'; memory.o:(.bss+0x4): first defined here

fix definition of the global variables nrow and ncol.

src/screen.c

@@ -36,6 +36,8 @@ static int indexX, indexY;
 static WINDOW *screen;
 static long long interval_start; /* interval start time in u sec */
 
+int nrow, ncol;
+
 char getch_screen(void)
 {
 	return (char)wgetch(screen);

src/screen.h

@@ -25,5 +25,5 @@ void outputDsp(unsigned char dsp);
 char getch_screen(void);
 void select_screen(void);
 
-int nrow, ncol;
+extern int nrow, ncol;