add missing case mapping between U+03F3 and U+037F

somehow this seems to have been overlooked. add it now so that
subsequent overhaul of case mapping implementation will not introduce
a functional change at the same time.

src/ctype/towctrans.c

@@ -176,6 +176,7 @@ static const unsigned short pairs[][2] = {
 	{ 0x243, 0x180 },
 	{ 0x244, 0x289 },
 	{ 0x245, 0x28c },
+	{ 0x37f, 0x3f3 },
 	{ 0x3f4, 0x3b8 },
 	{ 0x3f9, 0x3f2 },
 	{ 0x3fd, 0x37b },