Fix memory leak in srcpos_verror()

Since dtc runs are short, we don't care that much about memory leaks.
Still, leaking the source position string every time we print an error
messages is pretty nasty.  Fix it.

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

srcpos.c

@@ -293,13 +293,15 @@ srcpos_string(struct srcpos *pos)
 void
 srcpos_verror(struct srcpos *pos, const char *fmt, va_list va)
 {
-	const char *srcstr;
+	char *srcstr;
 
 	srcstr = srcpos_string(pos);
 
 	fprintf(stderr, "Error: %s ", srcstr);
 	vfprintf(stderr, fmt, va);
 	fprintf(stderr, "\n");
+
+	free(srcstr);
 }
 
 void