Fix warnings that would be caused by gcc flag -Wwrite-strings

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5206 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/exec.c b/exec.c
index 5e30689..18f46cd 100644
--- a/exec.c
+++ b/exec.c
@@ -180,7 +180,7 @@
 #endif
 
 /* log support */
-char *logfilename = "/tmp/qemu.log";
+const char *logfilename = "/tmp/qemu.log";
 FILE *logfile;
 int loglevel;
 static int log_append = 0;