avdevice/decklink: Fix compilation of module on OSX

Clang applies the missing-prototypes warning on C++ files, whereas
gcc only applies it to C.  As a result, the decklink_common.cpp file
fails to build because of missing prototypes in DecklinkDispatch.cpp
(which is #included by decklink_common.cpp).

We don't want to change the actual Blackmagic SDK sources, so
suppress the warning just for that one #include.

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavdevice/decklink_common.cpp

@@ -29,7 +29,18 @@ extern "C" {
 #ifdef _WIN32
 #include <DeckLinkAPI_i.c>
 #else
+/* The file provided by the SDK is known to be missing prototypes, which doesn't
+   cause issues with GCC since the warning doesn't apply to C++ files.  However
+   Clang does complain (and warnings are treated as errors), so suppress the
+   warning just for this one file */
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+#endif
 #include <DeckLinkAPIDispatch.cpp>
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
 #endif
 
 extern "C" {