Cleanup includes

Improved manually with the help of neovim LSP warnings and iwyu:

    iwyu -Ibuilddir/app/ -Iapp/src/ app/src/XXX.c

app/src/util/vector.h

@@ -5,8 +5,8 @@
 
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 #include <stdlib.h>
-#include <string.h>
 
 // Adapted from vlc_vector:
 // <https://code.videolan.org/videolan/vlc/-/blob/0857947abaed9c89810cd96353aaa1b7e6ba3b0d/include/vlc_vector.h>