Make "msg" const in internal_vproblem

I noticed that the "msg" variable in internal_vproblem could be
"const".  This seems like an improvement because it can wind up in
rodata.

Tested by rebuilding.

gdb/ChangeLog
2019-04-11  Tom Tromey  <tromey@adacore.com>

	* utils.c (internal_vproblem): Make "msg" const.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2019-04-11  Tom Tromey  <tromey@adacore.com>
+
+	* utils.c (internal_vproblem): Make "msg" const.
+
 2019-04-11  Alan Hayward  <alan.hayward@arm.com>
 
 	* aarch64-tdep.c (aarch64_analyze_prologue_test): Reset saved regs.

gdb/utils.c

@@ -300,7 +300,7 @@ internal_vproblem (struct internal_problem *problem,
 
   /* Don't allow infinite error/warning recursion.  */
   {
-    static char msg[] = "Recursive internal problem.\n";
+    static const char msg[] = "Recursive internal problem.\n";
 
     switch (dejavu)
       {