Fix bug that was revealed by arm cross-compilation.

Review URL: http://codereview.chromium.org/1519032
diff --git a/tpmd/unix/tpmd.c b/tpmd/unix/tpmd.c
index 061ef48..0194a23 100644
--- a/tpmd/unix/tpmd.c
+++ b/tpmd/unix/tpmd.c
@@ -171,7 +171,7 @@
 
 static void parse_options(int argc, char **argv)
 {
-    char c;
+    int c;
     struct passwd *pwd;
     struct group *grp;
     opt_uid = getuid();