dtc: Mark various internal functions static

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/scripts/dtc/treesource.c b/scripts/dtc/treesource.c
index ebeb6eb..1521ff1 100644
--- a/scripts/dtc/treesource.c
+++ b/scripts/dtc/treesource.c
@@ -52,7 +52,7 @@
 		fputc('\t', f);
 }
 
-int isstring(char c)
+static int isstring(char c)
 {
 	return (isprint(c)
 		|| (c == '\0')