Add #include <cstdint> to fix building with gcc 15 (#2174)

With gcc 15, the C++ Standard Library no longer includes other headers
that were internally used by the library. In conky's case the missing
header is `<cstdint>`

Downstream Gentoo bug: https://bugs.gentoo.org/939667

Signed-off-by: Kostadin Shishmanov <kostadinshishmanov@protonmail.com>

src/content/colours.hh

@@ -34,6 +34,7 @@
 
 #include <cassert>
 #include <climits>
+#include <cstdint>
 #include <memory>
 #include <string>
 #include <unordered_map>