core: drop unnecessary __useless_struct_to_allow_trailing_semicolon__

ISO C does not allow empty statements outside of functions, and gcc
will warn the trailing semicolons when compiling with -pedantic:

  warning: ISO C does not allow extra ‘;’ outside of a function [-Wpedantic]

But our code cannot compile with -pedantic anyway, at least because

  warning: ISO C does not support ‘__PRETTY_FUNCTION__’ predefined identifier [-Wpedantic]

Without -pedatnic, clang and even old gcc (3.4) generate no warnings about
those semicolons, so let's just drop __useless_struct_to_allow_trailing_semicolon__.

src/basic/macro.h

@@ -428,7 +428,6 @@ static inline unsigned long ALIGN_POWER2(unsigned long u) {
         static inline void func##p(type *p) {                   \
                 if (*p)                                         \
                         func(*p);                               \
-        }                                                       \
-        struct __useless_struct_to_allow_trailing_semicolon__
+        }
 
 #include "log.h"