libobs,obs-outputs: Fix librtmp1 interference

Details in #6226, basically this fixes an issue where OBS links
to the system-installed librtmp1, which is incompatible with our
librtmp.

Co-authored-by: ewhac <ewhac@ewhac.org>

libobs/util/c99defs.h

@@ -50,7 +50,7 @@
 #ifdef _MSC_VER
 #define EXPORT __declspec(dllexport)
 #else
-#define EXPORT
+#define EXPORT __attribute__((visibility("default")))
 #endif
 
 #include <stddef.h>