vk_compute_pass: Explicitly cast to VkAccessFlags

According to the standard, a narrowing conversion is an implicit conversion from an integer or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except when the value is a literal or constant expression.
MSVC, unlike GCC or Clang, determines this to be a narrowing conversion despite the enumeration exclusively containing values that fit within the range of a 32 bit integer, emitting a warning since designated initializers prohibit narrowing conversions.
To solve this, explicitly cast to the type we are initializing.