sdk: make C++11 plugins build again

compositor.h must not define a 'static_assert' macro, since that
conflicts with the new 'static_assert' in the standard and breaks
the build.

src/compositor.h

@@ -45,10 +45,6 @@ extern "C" {
 
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 #define container_of(ptr, type, member) ({				\
 	const __typeof__( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})