Change static_assert from one to two arguments.

Static_assert with 1 or 2 arguments is supported since gcc 6, older gcc 5 doesn't support static_assert with 1 argument:

In file included from /home/lekto/programy/orbital/src/compositor/global.h:23:0,
                 from /home/lekto/programy/orbital/src/compositor/compositor.h:29,
                 from /home/lekto/programy/orbital/src/compositor/main.cpp:26:
/home/lekto/programy/orbital/src/compositor/utils.h: In member function ‘void Signal<Args>::connect(T*, void (F::*)(FArgs ...))’:
/home/lekto/programy/orbital/src/compositor/utils.h:119:51: error: expected ‘,’ before ‘)’ token
         static_assert(std::is_base_of<F, T>::value);
                                                   ^
/home/lekto/programy/orbital/src/compositor/utils.h:119:51: error: expected string-literal before ‘)’ token

After change this line to:
static_assert(std::is_base_of<F, T>::value, "foo");
i succesfully build it with gcc 5.4 and run orbital.