Fix MIPS N32/N64 compile error

* syscall.c [LINUX_MIPSN32 || LINUX_MIPSN64] (syscall_enter): Pass
tcp->pid to ptrace().
diff --git a/syscall.c b/syscall.c
index 4f46bf6..a56d7df 100644
--- a/syscall.c
+++ b/syscall.c
@@ -2057,7 +2057,7 @@
 		else
 			nargs = tcp->u_nargs = MAX_ARGS;
 
-		if (ptrace (PTRACE_GETREGS, pid, NULL, (long) &regs) < 0)
+		if (ptrace (PTRACE_GETREGS, tcp->pid, NULL, (long) &regs) < 0)
 			return -1;
 
 		for(i = 0; i < nargs; i++) {