Remove pthread_exit().

Returning from the thread this way prevents local variables to be
destructed correctly since pthread_exit is marked noreturn.

This fixes a segmentation fault with glibc and llvm-libunwind on exit.

src/btop.cpp

@@ -657,7 +657,7 @@ namespace Runner {
 				<< Term::sync_end << flush;
 		}
 		//* ----------------------------------------------- THREAD LOOP -----------------------------------------------
-		pthread_exit(NULL);
+		return {};
 	}
 	//? ------------------------------------------ Secondary thread end -----------------------------------------------