Fix compilation error with clang in gdb/testsuite/gdb.trace/tspeed.c

Clang fails to compile the above file, with the following error:
  warning: using the result of an assignment as a condition without
  parentheses [-Wparentheses]

This prevents the following testcase from executing:
  gdb.trace/tspeed.exp

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-20  Gary Benson <gbenson@redhat.com>
+
+	* gdb/testsuite/gdb.trace/tspeed.c: Fix compilation error with
+	clang.
+
 2020-04-20  Gary Benson <gbenson@redhat.com>
 
 	* gdb.base/jit-main.c: Fix compilation error with clang.

gdb/testsuite/gdb.trace/tspeed.c

@@ -75,7 +75,7 @@ main(int argc, char **argv)
 
       /* Keep trying the speed test, with more iterations, until
 	 we get to a reasonable number.  */
-      while (problem = trace_speed_test())
+      while ((problem = trace_speed_test()))
 	{
 	  /* If iteration isn't working, give up.  */
 	  if (iters > max_iters)