math: fix align_up declaration

Using add() which is non-constexpr, the align_up() itself may not be
constexpr.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/stdex/math.hpp

@@ -89,7 +89,7 @@ namespace stdex
 	///
 	/// \return The first number greater or equal to a that has n lower bits zero
 	///
-	inline constexpr size_t align_up(size_t a, int n)
+	inline size_t align_up(size_t a, int n)
 	{
 		const size_t mask = SIZE_MAX << n;
 		return add(a, ~mask) & mask;