dtc: Make many functions 'static'

This patch marks various functions not shared between c files
'static', as they should be.  There are a couple of functions in dtc,
and many in the testsuite.

This is *almost* enough to enable the -Wmissing-prototypes warning.
It's not quite enough, because there's a mess of junk in the flex
generated code which triggers that warning which I'm not yet sure how
to deal with.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

treesource.c

@@ -52,7 +52,7 @@ static void write_prefix(FILE *f, int level)
 		fputc('\t', f);
 }
 
-int isstring(char c)
+static int isstring(char c)
 {
 	return (isprint(c)
 		|| (c == '\0')