Fix narrowing conversions for ppc

These constants are too large for `long long` so are unsigned,
and then cannot be narrowed to the signed type.

Fixes boostorg#29

include/boost/numeric/interval/detail/ppc_rounding_control.hpp

@@ -28,10 +28,10 @@ typedef union {
   double dmode;
 } rounding_mode_struct;
 
-static const rounding_mode_struct mode_upward      = { 0xFFF8000000000002LL };
-static const rounding_mode_struct mode_downward    = { 0xFFF8000000000003LL };
-static const rounding_mode_struct mode_to_nearest  = { 0xFFF8000000000000LL };
-static const rounding_mode_struct mode_toward_zero = { 0xFFF8000000000001LL };
+static const rounding_mode_struct mode_upward      = { (::boost::long_long_type)0xFFF8000000000002LL };
+static const rounding_mode_struct mode_downward    = { (::boost::long_long_type)0xFFF8000000000003LL };
+static const rounding_mode_struct mode_to_nearest  = { (::boost::long_long_type)0xFFF8000000000000LL };
+static const rounding_mode_struct mode_toward_zero = { (::boost::long_long_type)0xFFF8000000000001LL };
 
 struct ppc_rounding_control
 {