gdb/testsuite/rocm: fix rocm-multi-inferior-gpu.cpp

The gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp testcase contains a
call to execl which does not have NULL as a last argument.  This is
an invalid use of execl.  This patch fixes this oversight.

Change-Id: I03b60abe30468d71ba5089b240c6d00f9b8883b2
Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp

diff --git a/gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp b/gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp
index ca86923..d64afdd 100644
--- a/gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp
+++ b/gdb/testsuite/gdb.rocm/multi-inferior-gpu.cpp
@@ -62,7 +62,7 @@
       if (pid == 0)
 	{
 	  /* Exec to force the child to re-initialize the ROCm runtime.  */
-	  if (execl (argv[0], argv[0], n) == -1)
+	  if (execl (argv[0], argv[0], n, nullptr) == -1)
 	    {
 	      perror ("Failed to exec");
 	      return -1;