include/vlc_plugin.h: fixed problematic linkage on callback argument

The rationale behind this patch is easier to explain with a little bit
of code than in words, but one can summarize it with; "wrong linkage
used for `vlc_set_cb` when `include/vlc_plugin.h` is compiled as C++,
this fixes that".

Explanation
-----------

    extern "C" typedef void(*callback_t)();

    void cpp_func (callback_t);

Above the name `cpp_func` has C++ linkage, and its type is a C++ function
returning `void`, accepting a pointer-to-function-with-C-linkage (returning
`void` and takes no arguments).

    typedef void(*callback_t) ();

    extern "C" int c_func (callback_t);

In this example (matching the code in `include/vlc_plugin.h`), the name `c_func`
has C linkage, and its type is a C function returning `int`, accepting a
pointer-to-function-with-C++-linkage (that returns `void` and takes no
arguments).

Conclusion
----------

Since `vlc_entry_*` will be called from C, the first parameter when invoked will
be a pointer to function with C linkage---as such this patch fixes the
previously erroneous linkage.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

include/vlc_plugin.h

@@ -234,7 +234,7 @@ enum vlc_module_properties
 #   define EXTERN_SYMBOL
 #endif
 
-typedef int (*vlc_set_cb) (void *, void *, int, ...);
+EXTERN_SYMBOL typedef int (*vlc_set_cb) (void *, void *, int, ...);
 
 #define vlc_plugin_set(...) vlc_set (opaque,   NULL, __VA_ARGS__)
 #define vlc_module_set(...) vlc_set (opaque, module, __VA_ARGS__)