AK: Don't forward declare abort.

There is no portable way to forward declare abort because the libc
implementations disagree on the signature.

Originally, I added a __portable_abort function with a "portable"
signature which just called abort. But I really don't like it and just
including <stdlib.h> is simpler.

Note that the headers we include in <AK/TestSuite.h> are no longer
commutative now, we have to include <stdlib.h> before anything else.

AK/TestSuite.h

@@ -28,8 +28,6 @@
 
 #define AK_TEST_SUITE
 
-extern "C" __attribute__((noreturn)) void abort() noexcept;
-
 namespace AK {
 
 template<typename... Parameters>
@@ -63,6 +61,8 @@ using AK::warnln;
         ::abort();                                                                               \
     } while (false)
 
+#include <stdlib.h>
+
 #include <AK/Format.h>
 #include <AK/Function.h>
 #include <AK/NonnullRefPtrVector.h>