fix another -Wall warning

diff --git a/procps/ps.c b/procps/ps.c
index cf939bb..1ab3973 100644
--- a/procps/ps.c
+++ b/procps/ps.c
@@ -493,7 +493,7 @@
 #if !ENABLE_FEATURE_PS_WIDE
 	enum { terminal_width = 79 };
 #else
-	int terminal_width;
+	unsigned terminal_width;
 	int w_count = 0;
 #endif