Mark several functions/variables static

Spotted by parsing the output of `gcc -Wmissing-prototypes [...]`.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index 54e659c..e199a92 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -153,7 +153,7 @@
 	return NULL;
 }
 
-void show_error(char *msg)
+static void show_error(char *msg)
 {
 	ctx.page.mimetype = "text/html";
 	cgit_print_http_headers(&ctx);