struct Foo * declares a new type (namely struct Foo)
if there is no declaration of struct Foo already visible
in the current scope; otherwise it is just a pointer to
an element of the already declared type "struct Foo".
There is a gotcha with the first case:
struct Foo is only declared in its scope; a later declaration
of struct Foo in an enclosing scope declares a different type.

This happens in hwcontext_vulkan.h if it is included before
hwcontext.h, because some declarations of struct AVHWDeviceContext
and struct AVHWFramesContext have function prototype scope.

Compilers warn about this (during checkheaders):
‘struct AVHWDeviceContext’ declared inside parameter list will not
be visible outside of this definition or declaration

Fix this by including hwcontext.h.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>