decklink: Add Linux DeckLink project

The code specific to Linux helps convert `const char *` instances to
`std::string`s.

plugins/decklink/linux/platform.cpp

@@ -0,0 +1,12 @@
+#include "../platform.hpp"
+
+bool DeckLinkStringToStdString(decklink_string_t input, std::string& output)
+{
+	if (input == nullptr)
+		return false;
+
+	output = std::string(input);
+	free((void *)input);
+
+	return true;
+}