When targeting the UWP API subset, the LoadLibrary function is not
available (and the fallback, LoadPackagedLibrary, can't be used to
load system DLLs). In these cases, link directly to the functions
in the DLLs instead of trying to load them dynamically at runtime.

Merges Libav commit fd1ffa1f.
Signed-off-by: Martin Storsjö <martin@martin.st>