um: Move syscall() declaration into os.h

Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/os.h

@@ -301,4 +301,6 @@ extern int get_pty(void);
 /* sys-$ARCH/task_size.c */
 extern unsigned long os_get_top_address(void);
 
+long syscall(long number, ...);
+
 #endif