Fix compiler warning

Introduced in 0aa8f7640.

MSVC warned about performing 32-bit bit shifting when it appeared like we
might like a 64-bit result.  We did, but it just so happened that none of
the calls to this function could have caused the 32-bit shift to overflow.

Here we just cast the constant to int64 to make the compiler happy.

Discussion: https://postgr.es/m/CAApHDvofA_vsrpC13mq_hZyuye5B-ssKEaer04OouXYCO5-uXQ@mail.gmail.com

src/backend/utils/adt/dbsize.c

@@ -627,7 +627,7 @@ numeric_shift_right(Numeric n, unsigned count)
 	Datum		divisor_numeric;
 	Datum		result;
 
-	divisor_numeric = NumericGetDatum(int64_to_numeric(1 << count));
+	divisor_numeric = NumericGetDatum(int64_to_numeric(((int64) 1) << count));
 	result = DirectFunctionCall2(numeric_div_trunc, d, divisor_numeric);
 	return DatumGetNumeric(result);
 }